Process for the manufacture of coarsely crystalline to monocrystalline sheets of semiconductor material

The invention provides a process for the manufacture of coarsely crystalline to monocrystalline sheets and/or plates of semiconductor material of preferred orientation. A meniscus of molten semiconductor material comes in contact with a moving, cooler substrate of the same coarsely crystalline to monocrystalline semiconductor material, during which, while transferring the preferred orientation, a thin sheet of the semiconductor material is pulled onto the substrate and, after cooling, becomes detached from the substrate. The substrate can be reused as often as desired.

The present invention relates to a process for the manufacture of coarsely 
crystalline to monocrystalline sheets and/or plates of semiconductor 
material of preferred orientation. More particularly, it relates to such a 
process wherein molten semiconductor material is applied to a substrate of 
the same coarsely crystalline to monocrystalline semiconductor material 
and, after solidifying, becomes detached from the substrate as a result of 
thermal stress. 
Solar cells, as used in space travel as current generators, are far too 
expensive to be used widely on earth. The main reason for their high cost 
is the manufacturing process, which requires a large amount of work and 
material, and according to which monocrystalline silicon rods or bars 
obtained by crucible-pulling or zone-pulling are sawn up, with a 
considerable loss of material, to form monocrystalline silicon wafers. 
The avoidance of this expensive and very wasteful sawing step has been the 
aim of numerous processes which have become known in the meantime and in 
which attempts have been made to obtain silicon directly in plate or sheet 
form. This is effected, for example, by pulling a monocrystalline silicon 
band from a polycrystalline silicon supply rod via a shaping die. 
According to the EFG-process, (edge film-fed growth process), 
monocrystalline silicon is pulled off in an upward direction, in band 
form, for example via a shaping capillary body of carbon dipped into a 
silicon melt. According to Bleil's process, silicon is melted in a 
crucible and a silicon band is pulled off sideways by means of a seed 
crystal under the effect of a temperature gradient, the level of the melt 
in the crucible being kept constant by a system of displacement bodies 
dipped into the melt. Finally, according to Shockley's process, band-form 
silicon is obtained by melting a polycrystalline supply rod onto liquid 
lead and pulling the silicon horizontally, from the lead film, in the form 
of a band, by means of a seed crystal, and under the effect of a 
temperature gradient. The efficiency of the processes mentioned is, 
however, limited by the low pulling speeds, which are of the order of a 
few centimeters per minute, The last mentioned process is further 
complicated by the fact that the lead used as the substrate surface must 
be extremely pure so that the silicon which is melted and resolidified 
thereon is not contaminated. A further disadvantage is the high vapor 
pressure of the lead, the result of which is that lead is inevitably 
deposited on the silicon band at the cool end of the apparatus from which 
the resolidified silicon is pulled off. A comprehensive description of 
these and similar processes can be found in an article by Jean-Jaques 
Brissot, "Silicium pour photopiles solaires", Acta Electronica, 20, 2, 
1977, pages 101 to 116. 
The process according to DE-OS 29 03 061, according to which a silicon 
sheet is pulled from a melt embedded in a non-elemental slide melt, is 
technically difficult to carry out because of the necessary precise 
replacement and control of the slide melt, especially at the pull-off 
position. Above all, evaporation and the associated change in composition, 
aggravated by reactions between the crucible, the melt and the slide melt, 
can cause viscosity changes and thus constantly alter the sliding 
properties. 
Finally, according to the process described in DE-OS 28 30 522, silicon 
bands can be obtained by applying liquid silicon to a rotating 
monocrystalline silicon support, the bands which have grown being 
centrifuged off the support with material being stripped off. In this 
process, the support has to be maintained at a high temperature just below 
the melting point of silicon; in addition, because of the loss of 
material, the support can be used only for a limited time and has to be 
replaced from time to time. 
The problem of the invention was therefore to provide a process by means of 
which coarsely crystalline and monocrystalline semiconductor sheets can be 
manufactured at a high pulling speed with the aid of a substrate that can 
be reused as often as desired. 
This problem is solved by a process which is characterized in that molten 
semiconductor material is brought into contact with a substrate of the 
same coarsely crystalline to monocrystalline semiconductor material which 
is guided past the point of contact at a speed of at least 75 mm/s, and in 
that, in order to ensure the automatic release of the growing sheet or 
plate, the substrate is maintained at a temperature of not more than 0.8 
T.sub.M, where T.sub.M denotes the melting point of the semiconductor 
material expressed in degrees Kelvin. 
In this process, it has proved advantageous first to place the 
semiconductor material from which the sheet is to be produced, in a 
suitable form, for example in the form of a granulate in the case of 
silicon, into a melt-preparation vessel that is separate from the actual 
pulling system, and to melt it therein. The melt is then transferred via a 
connecting system. This can be carried out, for example, by means of an 
overflow system in conjunction with displacement bodies so that a uniform 
flow of the molten semiconductor material to the pulling system is 
ensured. 
A one-component material, such as, for example, silicon or germanium, is 
preferably used as the semiconductor material for the purposes of the 
invention, although the invention can also be applied to multi-component 
semiconductor materials, such as, for example, III-V compounds, for 
example, indium phosphide, gallium phosphide or gallium arenside. 
The pulling system contains the pulling crucible, which is made of graphite 
or, preferably, quartz. The molten semiconductor material, which has 
flowed from the melt-preparation vessel, is collected in the pulling 
crucible and adjusted to a temperature of up to 100.degree. C., but 
preferably of from 5.degree. to 50.degree. C., above its melting point. 
The crucible can be heated, for example, by resistance heating or 
induction heating. 
The melt and the substrate can be brought into contact with one another, 
for example, by pouring, centrifuging or spraying the melt onto the 
substrate. A further possibility is, for example, to so design one side of 
the pulling crucible so that the molten semiconductor material contained 
therein can be brought into contact with the substrate. To this end, it 
has proved to be especially advantageous to cause the molten semiconductor 
material to form a meniscus that projects beyond the edge of the crucible 
and is stabilized by surface tension. That can be achieved, for example, 
by simple control of the inclination of the crucible. It is, however, 
especially advantageous for one side of the pulling crucible to have a 
slot-like aperture-preferably at the level of the base of the crucible-of 
such dimensions that the molten semiconductor material can pass through it 
forming a meniscus. The formation of the meniscus is influenced, for 
example, by the melt temperature, the inclination of the crucible and the 
dimensions of the slot. 
The height of the slot-like aperture depends on the required stability of 
the resulting meniscus, while the width of the slot depends on the desired 
width of the sheet. Particularly good results can be obtained when the 
width of the slot is from 1 to 20 mm, preferably 10 mm, greater than the 
width of the substrate. 
The actual pulling of the sheet is carried out by bringing a substrate made 
of the same coarsely crystalline to monocrystalline semiconductor material 
into contact with the meniscus of molten semiconductor material and moving 
it past the meniscus at a speed of at least 75 mm/s. It has proved 
favorable for the free surface of the melt film pulled off at the contact 
position in that way to be maintained at the melt temperature by an 
additional heating device, for example an electron-beam device or a laser 
gun. In the course of this pulling operation, the preferred orientation of 
the substrate is imparted to the solidifying semiconductor material and 
thus coarsely crystalline to monocrystalline sheet is pulled onto the 
substrate, and that sheet then becomes detached, unaided, from the 
substrate as a result of thermal stress. 
In the case of continuous operation, the substrate can be moved past the 
meniscus in the form of an "endless band" or a cylinder, for example. In 
the case of semi-continuous operation, it is also possible to use, as the 
substrate, individual ramps having a length of, for example, 1 m. The 
width of the particular substrate depends on the desired width of the 
sheet or plate. It has, however, in all cases, proved advantageous to make 
up the substrates from individual elements that have the same dimensions 
as the desired product, for example, solar cells having a length of 100 mm 
and a width of 100 mm. Numerous other variations in shape and size are, 
however, also possible. 
Particularly good results can be achieved if the movement of the substrate 
past the meniscus is in an upward direction and deviates from the vertical 
by from 0.degree. to 60.degree., preferably from 0.degree. to 20.degree.. 
If the distance between the individual elements is small, for example 
.ltoreq.0.05 mm, a sheet is obtained that is not sub-divided, and which 
can then be divided into the desired individual pieces by suitable process 
steps, for example, sawing or scribing and then breaking up. If, however, 
there is a large distance between the individual elements, for example 
from 0.5 to approximately 2 mm, sheets are obtained that are already 
separated into pieces corresponding to the size of the individual 
elements, and thus the dividing step described above is unnecessary. 
One factor that is important for achieving a coarsely crystalline to 
monocrystalline growth and the automatic release of the sheet from the 
substrate is the temperature of the substrate. If the temperature is too 
low, the growing sheet contains many microcrystalline areas unfavorable 
for solar cell base material. If the temperature of the substrate is too 
high, the sheet will grow in the manner of liquid epitaxy; automatic 
release of the sheet from the substrate will therefore no longer be 
possible. A temperature difference between the melt and the substrate of 
approximately from 0.2 to 0.5 T.sub.M, preferably of approximately from 
0.3 to 0.4 T.sub.M, has proved particularly suitable for carrying out the 
process according to the invention; for silicon, for example, the 
temperature difference may be approximately from 350 to 850 K, preferably 
from 500 to 680 K, with T.sub.M, the melting temperature of the silicon, 
having a value of 1693 K. Ihe substrate is advantageously adjusted to the 
desired temperature in a temperature-adjusting tunnel that can be 
controlled separately. 
An important factor in carrying out the process according to the invention 
is the pulling speed at which the substrate is moved past the position of 
contact with the molten semiconductor material. For reasons of 
effectiveness alone, the highest possible values are desired. Thus, when 
using silicon in the process according to the invention, speeds of over 
300 mm/s have already been reached. In general, for example, it is 
possible to promote an increase in the pulling speed by increasing the 
temperature difference between the substrate and the melt, but there is 
the limitation that, above certain limit values, the crystal structure is 
no longer transferred satisfactorily from the substrate to the sheet. 
In addition to the temperature and the pulling speed, the type of surface 
of the substrate also plays a part in the process according to the 
invention. As a rule of thumb, it may be stated that a low degree of 
surface roughness facilitates the automatic release of the sheet which has 
grown. 
Other objects and features of the present invention will become apparent 
from the following detailed description considered in connection with the 
accompanying drawings, which disclose several embodiments of the 
invention. It is to be understood that the drawings are to be used for the 
purpose of illustration only, and not as a definition of the limits of the 
invention.

Referring now in detail to the drawings, and in particular FIG. 1, a 
container 1 is shown which is made, for example, of refined steel, and 
which is advantageously double-walled in order to accommodate a cooling 
medium, such as, for example, water. The container 1 can be supplied, for 
example, with silicon granulate, by means of a refilling device 2. 
Inside container 1, there is arranged a melt-preparation crucible 3 inside 
a heating apparatus 4. This crucible is used as a supply vessel; its 
molten contents can be transferred, for example with the aid of one or 
more displacement bodies 5, via an overflow system 6, into a pulling 
crucible 7. Instead of using displacement bodies as shown, the overflow of 
the melt can also take place, for example, by means of a capillary body 
or, if a closed melt-preparation crucible that has an outlet aperture 
leading into the pulling crucible is used, by forcing the melt over by 
means of gas pressure. 
The pulling crucible, heated by the crucible-pulling heating system 8 by 
irradiation, induction or resistance heating has, at its front end, a 
slot-like aperture 9 from which the molten semiconductor material can be 
discharged to form a meniscus stabilized by surface tension. The meniscus 
is brought into contact with the substrate 10 which consists, for example, 
of a row of individual plates and which moves past the meniscus in an 
upward direction, a sheet 11 being applied to the substrate as a thin 
layer. The free surface of the melt film of the sheet 11, which film is 
pulled off at the contact position, can be maintained at the desired 
temperature by means of the supplementary heating system 12, preferably by 
means of radiant heating, e.g., laser irradiation. The formation of the 
meniscus before contact, and the formation of the free surface of the melt 
film pulled off at the contact position, which takes place during the 
application operation, can be monitored, for example by an optical control 
device 13. The beginning and end of the application operation can be 
particularly advantageously controlled if the pulling crucible can be 
tilted, thus making it possible to interrupt or bring about contact 
between the melt and the substrate in a simple manner. 
As early as a few seconds after complete crystallization, the connection 
between the sheet and the substrate, which is firm at the moment of 
application, begins to loosen. The sheet leaves the container through the 
outlet valve 14 and can be further processed, e.g., by sawing, into 
individual plates while the substrate is returned for reuse and is 
readjusted to the desired temperature in the temperature adjusting tunnel 
15. An inert gas atmosphere of, e.g., nitrogen, carbon dioxide, noble 
gases or mixtures of different inert gases may be maintained in the 
container; operation in a vacuum is, however, also possible. 
According to FIG. 2, the sheet can alternatively be pulled on a substrate 
designed as a ramp. For the sake of clarity, FIG. 2 does not show the 
container, the melt-preparation device or the melt-transfer system, since 
they can be designed, for example, as in the arrangement described in FIG. 
1. 
A pulling crucible 20, which is preferably of quartz, contains the melt 21 
of semiconductor material, maintained at a temperature of at least 
5.degree. C. above the melting temperature by means of a heating device 
22, for example a graphite melt heater. At the front end of the pulling 
crucible, a slot-like aperture 23 is provided, from which a meniscus 24 of 
the molten semiconductor material stabilized by surface tension projects 
by at least 0.5 mm. 
In a separately controllable temperature-adjusting tunnel 25, a substrate 
in the form of a movable ramp 26 is maintained at a temperature set within 
a range of from 0.5 to 0.8 T.sub.M, i.e., in the case of silicon, e.g., 
within a temperature range of from 850 to 1350 K, preferably from 1000 to 
1200 K (values rounded off). The ramp consists of coarsely crystalline to 
monocrystalline structural members 27 of the same semiconductor material 
as that contained in molten form in the pulling crucible. Those structural 
members are advantageously mounted on a slide 28 of suitable carrier 
material, for example graphite. By means of a guide means 29 and a lifting 
device 30, the slide can be moved out of the temperature-adjusting tunnel 
and past the pulling crucible in an upward direction at an accurately 
maintained distance from the front end of the pulling crucible. As this is 
done, first the ramp edge 31, and then the ramp surface 32, comes into 
contact with the meniscus of the semiconductor material and they are 
covered with a sheet that is coarsely crystalline to monocrystalline 
depending on the ramp material and that, even only shortly after 
solidifying, adheres only weakly to the ramp and can be readily removed, 
e.g., by means of suitable gripping devices, and further processed to form 
solar cell material. 
If the distance between the individual structural members that make up the 
ramp is sufficiently large, there is obtained, not a continuous sheet, but 
pieces of sheet that have already become separated from one another, for 
example, plates having the dimensions of the structural members. The 
choice of the ramp temperature and the speed at which the ramp is moved 
can, in a simple manner, also determine the thickness of the sheet 
obtained, so that it is possible, in a single operation, to manufacture 
pieces of semiconductor material having the desired width, length and 
thickness. 
By using a series of ramps arranged, for example, in the form of a circle 
or an endless band, which are moved in succession past the meniscus, this 
process can advantageously also be carried out continuously, since the 
ramps can be reused as often as desired. 
Thus, the process according to the invention, provides an efficient method 
of manufacturing semiconductor material for inexpensive solar cell base 
material. 
The invention will now be described by several examples which are given by 
way of illustration and not of limitation. 
EXAMPLE 1 
Using an apparatus as shown in FIG. 1, silicon is continuously melted in a 
melt-preparation crucible, which is a partitioned crucible, and the 
silicon is continuously supplemented, at a rate of approximately 21 g/s, 
in the form of a granulate having a grain size of from 1 to 5 mm. Instead 
of being in granular form, the silicon can also be introduced into the 
melt-preparation crucible by direct melting of a polysilicon rod or, 
alternatively, it can be introduced directly into the pulling crucible if 
the quantity melted is suitably controlled. 
The melt, which is maintained at 1430.degree. C. in the melt preparation 
crucible, is transferred into pulling crucible by displacement bodies in 
accordance with the quantity of silicon subsequently added, and is there 
adjusted to a melt level of approximately 11 mm in the case of a 
horizontal crucible position. The pulling crucible has a slot-like 
aperture 6 mm high and 110 mm wide through which the molten silicon 
passes, forming a meniscus stabilized by surface tension. The temperature 
at the overhanging meniscus, which measures approximately 1 mm, is 
maintained at 1430.degree. C. by laser heating. 
The substrate used in the process according to the invention consists of 
ninety 10 mm-thick monocrystalline silicon plates of preferred orientation 
and each measuring 100.times.100 mm and having a surface roughness depth 
of approximately from 1 to 2 .mu.m. The plates are circulated in such a 
manner that, from a position approximately 250 mm upstream of the 
melt/substrate contact position and for a distance of 500 mm, they xove in 
an upward direction past the pulling crucible at a distance of 0.5 mm 
therefrom, in the manner of an "endless band" and at a speed of 300 mm/s. 
During this operation, the direction of the "endless band" deviates by 
approximately 20.degree. from the vertical; the distance of the plates 
from one another is .ltoreq.0.05 mm. After passing through this guide 
area, the silicon plates are slowed to a speed of approximately 35 mm/s, 
they pass through a temperature-adjusting tunnel, where they are adjusted 
to a temperature of 680.degree. C., and finally they return to the area 
where they are guided as an "endless band". 
The silicon sheet is applied to the moving substrate by bringing the latter 
into contact with the meniscus of the molten silicon by tilting the 
pulling crucible through approximately 2.degree. . The sheet applied in 
this manner detaches itself unaided from the substrate immediately after 
cooling to the temperature of the substrate. The substrate is returned to 
the temperature-adjusting tunnel and is there adjusted to a temperature of 
680.degree. C. The detached 0.3 mm thick sheet is guided out of the 
container, which has been evacuated to approximately 10.sup.- mbar, 
through the vacuum valve and is divided up by laser scribing into 
monocrystalline plates 100 mm long and 100 mm wide, which have the 
preferred orientation of the substrate. 
EXAMPLE 2 
Using an apparatus as shown in FIG. 2, 85 g of silicon were melted and 
gradually transferred into the pulling crucible where the melt was 
adjusted to a temperature of 1450.degree. C. The pulling crucible had a 
slot-like aperture 5 mm high and 60 mm wide through which a meniscus of 
molten silicon overhung by 2 mm. The temperature at the overhanging 
meniscus, measured by means of a radiation pyrometer, was 1450.degree. C. 
The substrate that was used in the process according to the invention, and 
which was in the form of a ramp, consisted of ten 50.times.50 mm 8 
mm-thick plates of coarsely crystalline silicon having an average 
crystallite size of from 5 to 10 mm, the surface having a roughness depth 
of approximately 2 .mu.m. The distance of the plates from one another was 
0.04 mm; the plates were secured to a slide of graphite. The entire ramp 
was first maintained at a temperature of 900.degree. C. in a 
resistance-heated temperature-adjusting tunnel. 
Then, by means of a lifting device and with accurate guiding, the ramp was 
moved vertically upwards at a speed of 80 mm/s past, and at a distance of 
1 mm from, the meniscus which had been discharged from the slot-like 
aperture of the pulling crucible. Beginning with the first contact between 
the ramp and the meniscus, the silicon plates making up the ramp were 
covered with a, at first liquid, but rapidly solidifying, silicon sheet. 
This sheet began to become detached as early as during the cooling from 
melt temperature to ramp temperature. Finally, held by adhesion forces, 
the sheet rested on the ramp and could be removed from the support. The 
resulting sheet was 500 mm long, 50 mm wide and 0.3 mm thick. It had the 
same coarsely crystalline structure as that of the silicon plates used as 
substrate. After removing the sheet, it was possible to use the ramp again 
for the sheet-pulling process. Throughout the entire process, an argon 
atmosphere was maintained in the container, at a pressure of 10 mbar. 
EXAMPLE 3 
In this example, the procedure was as indicated in Example 2, except that 
the ramp used consisted of ten 8 mm-thick monocrystalline silicon plates 
measuring 50.times.50 mm which were mounted at a distance of 0.8 mm from 
one another on a graphite slide corresponding to that used in Example 2. 
In this case, it was possible to obtain, by the pulling process, ten 
separate, almost completely monocrystalline, silicon plates having a 
preferred orientation and a length of 50 mm, a width of 50 mm and a 
thickness of 0.3 mm. 
EXAMPLE 4 
In the apparatus shown in FIG. 2, 120 g of germanium are melted and 
gradually transferred into the pulling crucible having a slot-like 
aperture 3 mm high and 60 mm wide. The melt is maintained at 950.degree. 
C. and forms a meniscus having an overhang of approximately 1 mm. 
The ramp used consists of six 50.times.50 mm, 4 mm-thick plates of 
monocrystalline germanium having a roughness depth of approximately 2 
.mu.m. The plates are secured to a slide of graphite, at a distance of 
0.04 mm from one another. The entire ramp is first maintained at 
575.degree. C. in a resistance-heated temperature-adjusting tunnel. 
The ramp is then moved at a speed of 120 mm/s past the pulling crucible and 
at a distance of 0.5 mm therefrom, according to the process described in 
Example 2, and a germanium sheet is pulled off and finally removed from 
the support, which can be reused. 
The resulting monocrystalline germanium sheet of preferred orientation is 
300 mm long, 50 mm wide and 0.25 mm thick. Throughout the entire process, 
an argon atmosphere is maintained in the container at a pressure of 10 
mbar. 
While only several embodiments and examples have been shown and described, 
it will be obvious that many changes and modifications may be made 
thereunto, without departing from the spirit and scope of the invention.