U.S. patent application number 11/392372 was filed with the patent office on 2006-08-17 for method for making thin film devices intended for solar cells or silicon-on-insulator (soi) applications.
Invention is credited to Renat Bilyalov, Jef Poortmans, Chetan Singh Solanki.
Application Number | 20060184266 11/392372 |
Document ID | / |
Family ID | 29797373 |
Filed Date | 2006-08-17 |
United States Patent
Application |
20060184266 |
Kind Code |
A1 |
Solanki; Chetan Singh ; et
al. |
August 17, 2006 |
Method for making thin film devices intended for solar cells or
silicon-on-insulator (SOI) applications
Abstract
In one inventive aspect, a thin film device is manufactured by
(a) forming a porous semiconductor layer in the form of a thin film
on an original substrate, the formation being immediately followed
by (b) separation of the thin film by a lift-off process from the
original substrate; (c) transfer of the thin film to a dummy
support, the thin film not being attached to the dummy support; (d)
fabrication of a device on top of the thin film; and (e) transfer
and attachment of said device on said thin film on a foreign
substrate.
Inventors: |
Solanki; Chetan Singh;
(Leuven, BE) ; Bilyalov; Renat; (Tielt-Winge,
BE) ; Poortmans; Jef; (Kessel-Lo, BE) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET
FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Family ID: |
29797373 |
Appl. No.: |
11/392372 |
Filed: |
March 29, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10627576 |
Jul 24, 2003 |
7022585 |
|
|
11392372 |
Mar 29, 2006 |
|
|
|
Current U.S.
Class: |
700/121 ;
257/E21.567; 257/E21.57; 700/117; 700/119 |
Current CPC
Class: |
H01L 31/18 20130101;
H01L 21/76259 20130101; H01L 21/76251 20130101 |
Class at
Publication: |
700/121 ;
700/117; 700/119 |
International
Class: |
G06F 19/00 20060101
G06F019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 24, 2002 |
EP |
EP 02447146 |
Claims
1. A system for fabricating a thin film device comprising: an
apparatus for transferring a thin semiconductor layer to a dummy
substrate, the thin semiconductor layer not being attached to the
dummy substrate; and an apparatus configured to fabricate a device
on the thin semiconductor layer after the thin semiconductor layer
is transferred to the dummy substrate.
2. The system of claim 1, further comprising an apparatus for
forming the thin semiconductor layer on a substrate so that the
thin semiconductor layer is free-standing on the substrate.
3. The system according to claim 2, further comprising an apparatus
configured to separate the thin semiconductor layer from the
substrate.
4. The system of claim 1, wherein the fabricating apparatus
fabricates an epitaxial silicon layer on the thin semiconductor
layer.
5. The system of claim 1, wherein the apparatus configured to
fabricate a device is adapted for fabricating a first device on a
first side of the thin semiconductor layer and for fabricating a
second device on a second side of the thin semiconductor layer.
6. The system of claim 1, wherein the apparatus configured to
fabricate a device is adapted for fabricating at least one of a
front and a back side of a solar cell on the thin semiconductor
layer.
7. The system of claim 1, wherein the fabricating apparatus
deposits an active semiconductor layer on the thin semiconductor
layer.
8. The system according to claim 7, wherein deposition of the
active semiconductor layer is performed by epitaxial Chemical Vapor
Deposition.
9. The system of claim 1, further comprising an apparatus for
transferring the thin semiconductor layer, comprising the device,
to a foreign substrate.
10. The system of claim 9, further comprising an apparatus for
bonding the thin semiconductor layer to the foreign substrate.
11. The system of claim 1, wherein the device is a non-finished
device that is further finished after attachment to a foreign
substrate.
12. The system of claim 1, wherein the thin semiconductor layer is
separated from the substrate by immersing the substrate in a HF
solution in concentration between about 12 and 35% and using
current densities between about 50 and 250 mA/cm.sup.2.
13. The system of claim 1, wherein the thin semiconductor layer is
a double layer of crystalline or amorphous semiconductor material
including silicone germanium, III-V materials such as Ga As, InGaAs
and semiconducting polymers.
14. The system of claim 9, wherein the foreign substrate comprises
a low-cost substrate.
15. The system of claim 1, wherein the thin film device comprises a
solar cell.
16. The system of claim 9, wherein the foreign substrate comprises
glass.
17. A system for manufacturing a thin film device comprising: means
for transferring a thin semiconductor layer to a dummy substrate,
the thin semiconductor layer not being attached to the dummy
substrate; and means for fabricating a device on the thin
semiconductor layer after being transferred to the dummy
substrate.
18. The system of claim 17, further comprising means for forming
the thin semiconductor layer on a substrate so that the thin
semiconductor layer is free-standing on the substrate.
19. The system of claim 18, further comprising means for separating
the thin semiconductor layer from the substrate.
20. The system of claim 17, further comprising means for attaching
the device and the thin semiconductor layer on a foreign
substrate.
21. The system of claim 17, further comprising means for bonding
the thin semiconductor layer to the foreign substrate.
22. The system of claim 17, wherein the fabricating means comprises
means for fabricating an epitaxial silicon layer.
23. The system of claim 17, wherein the fabricating means is
adapted for fabricating a first device on a first side of the thin
semiconductor layer and for fabricating a second device on a second
side of the thin semiconductor layer.
24. The system of claim 17, wherein the fabricating means is
adapted for fabricating at least one of a front and a back side of
a solar cell on the thin semiconductor layer.
25. The system of claim 17, wherein the fabricating means deposits
an active semiconductor layer on the thin semiconductor layer.
26. The system according to claim 25, wherein deposition of the
active semiconductor layer is performed by epitaxial Chemical Vapor
Deposition.
27. The system of claim 17, wherein the device is a non-finished
device that is further finished after attachment to a foreign
substrate.
28. The system of claim 17, wherein the thin semiconductor layer is
separated from the substrate by immersing the substrate in a HF
solution in concentration between about 12 and 35% and using
current densities between about 50 and 250 mA/cm.sup.2.
29. The system of claim 1, wherein the thin semiconductor layer is
a porous semiconductor layer.
30. The system of claim 17, wherein the thin semiconductor layer is
a porous semiconductor layer.
31. The system of claim 1, further comprising means for clamping
the thin semiconductor layer on the dummy substrate.
32. The system of claim 17, further comprising means for clamping
the thin semiconductor layer on the dummy substrate.
33. A system for fabricating a thin film device, the system being
configured to form a thin semiconductor layer on a substrate so
that the thin semiconductor layer is free-standing on the
substrate, separate the thin semiconductor layer from the
substrate, and fabricate a device on the thin semiconductor layer
after the thin semiconductor layer is separated from the substrate.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 10/627,576, filed Jul. 24, 2003 and claims the benefit of
European Application No. EP02447146, filed Jul. 24, 2002, which are
hereby incorporated by reference in their entirety herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention is related to micro-electronics and
more particular to the field of thin film device applications such
as silicon-on-insulator (SOI) structures or solar cells in
particular. Furthermore, the present invention is relating to a
manufacturing method of such devices.
[0004] 2. Description of Related Technology
[0005] The silicon in existing semiconductor devices usually has a
thickness of several hundred microns. However, the electrically
active domain of a wafer is limited to its surface; in fact, less
than a few microns of thickness is needed. The remaining portion of
the wafer is used as substrate. Unfortunately, this excess of
material causes both a rise in power consumption and a fall in the
operating speed of the device. The SOI wafers incorporate an
insulating layer between its very thin (less than a few microns)
active domain and its much thicker substrate. The substrate is
isolated and can thus no longer deteriorate the speed or efficiency
of the active layer.
[0006] Silicon on insulator technology (SOI) involves the formation
of a monocrystalline silicon semi-conductor layer on an insulating
material such as silicon oxide.
[0007] One important application of a thin film device is the
manufacturing of solar cells. Solar cells usually comprise an
active surface on the top of a silicon wafer in the form of a thin
film device deposited on said silicon wafer.
[0008] During the conversion of light into electrical energy, as
mentioned above, only the top few microns of said top layer are
really active. The major part of this silicon wafer only provides
mechanical strength to the device. This function can be achieved by
any other low-cost substrate compatible to the production process.
The requirements for such a substrate, excepted low cost, are high
temperature stability (1100.degree. C.), matching of the thermal
expansion coefficients and low impurity contents.
[0009] More generally, in the prior art, the preparation of a
porous semiconductor layer on a substrate as a sacrificial layer
for solar cell usually comprises several steps such as at least a
porous semiconductor layer formation on an original substrate,
epitaxial silicon layer deposition, device fabrication on said
substrate and separation of the device from the original substrate
and transfer to a foreign substrate in order to possibly re-use the
original substrate. This sequence is largely illustrated in the
documents U.S. Pat. No. 6,258,698 (Iwasaki et al, Canon), U.S. Pat.
No. 6,211,038 (Nakagawa et al, Canon) and U.S. Pat. No. 6,326,280
(Tayanaka, Sony Corporation).
[0010] In the prior art, several methods are known to separate thin
(porous) semiconductor films from a substrate. All those methods
use a lift-off or peeling-off process at the end of the production
chain. The drawback of these methods is that during all the process
steps, parameters such as temperature, pressure and chemicals are
conditioned by the resistance of the original substrate. The film
separation and its transfer is the last technological step that
requires preserving the high-porous characteristics of the Si
layer. The fact of maintaining said porous characteristics
throughout many high-temperature steps allows only a narrow
processing window in terms of process temperature and porosity.
Moreover in said case, the transfer is difficult to achieve
properly.
[0011] In particular, a lift-off process is described in
EP-A-1132952 where it is shown that a thin porous silicon film of 5
to 50 .mu.m can be separated from the silicon substrate whereon it
is deposited. In such case, the substrate can be re-used many times
for getting new porous silicon films. Other possible techniques for
thin film separation are ion implantation or wafer bonding
techniques.
[0012] In EP-A-0993029, a method is disclosed for the production of
a crystalline semiconductor film. This is done by forming a porous
layer on a semiconductor substrate, lifting-off the porous layer,
and either before or after the lifting-off applying a thermal
annealing step such that the porous layer is at least partially
recrystallized. For the lift-off step a method is disclosed in
which the porous layer is attached to a `Hilfstrager`, which can be
translated as a `sub-carrier`, or a `foreign substrate`. The porous
layer is physically bonded or glued to said foreign substrate.
SUMMARY OF CERTAIN INVENTIVE ASPECTS
[0013] Various inventive aspects aim to provide an easy method for
the preparation of thin-film devices for structures being highly
efficient and low-cost. Examples of such structures are
silicon-on-insulator (SOI) structures or solar cells. The use of
thin film in SOI structures in general and in solar cells in
particular allows the reduction of the amount of material consumed
per structure, which significantly reduces the high costs of the
active substrate, while the quality of the film provides the good
characteristics of the whole device.
[0014] Further inventive aspects may reduce the impact of device
processing steps on the original substrate, porous silicon layer,
porous silicon layer formation, necessary glues and target
substrate.
[0015] Yet further inventive aspects may improve control on the
production-and-transfer process of thin films.
[0016] Several documents listed below are incorporated by reference
herewith. In particular, the document EP-A-1132952 is hereby
incorporated by reference as a whole, especially with respect to
the production of the thin porous film.
[0017] In the art, the term `original substrate` has also been
named `mother substrate`, `start-substrate` or `originating
substrate`. The term `dummy support` in the art can also be named
`intermediate support or substrate`, `support substrate`, `dummy
substrate`. The term `foreign substrate` in the art can also be
named `target substrate`, `final substrate` or `end-substrate`. The
foreign substrate can be any substrate, e.g., glass. More examples
of possible substrates are given in EP-A-0767486 and in U.S. Pat.
No. 6,391,219, where the final substrate is called support
substrate. To avoid any confusion, in the present context, the
distinction between the dummy support and the final support is that
the dummy support is a support on which the fabrication of a device
can easily take place with full freedom of process parameters
and/or without a negative impact of any device processing steps on
the original substrate, porous silicon layer, porous silicon layer
formation, necessary glues and/or target substrate. The dummy
support is a support to which the porous layer is transferred
intermediately without physical attachment thereto. The dummy
substrate provides the necessary support and/or mechanical strength
to the porous layer(s) during fabrication thereon of a device. In
the absence of such supportive material, the porous layer(s) might
break during device fabrication because too fragile.
[0018] One aspect of the present invention relates to a method for
manufacturing a semiconductor device. The method may comprise the
following: [0019] (a) formation of a porous semiconductor layer in
the form of a thin film on an original substrate, said formation
being immediately followed by the step of [0020] (b) separation of
said thin film by a lift-off process from said original substrate;
[0021] (c) the transfer of said thin film to a dummy support, said
thin film not being attached to said dummy support; [0022] (d)
fabrication of a device on top of said thin film; [0023] (e)
transfer and attachment of said device on said thin film on a
foreign substrate.
[0024] In one aspect, the method differs from methods known in the
art in the sequence of its steps. It further differs from methods
in the prior art by the fact that a device is fabricated on the
thin film while placed on a support to which the thin film is not
physically bonded and/or glued. In other words, the device is
fabricated on a free-standing thin film. The advantageous effects
that accompany these changes with respect to the prior art are
discussed throughout the specification.
[0025] How to perform the different steps per se is known in the
art. For instance, several possible methods for steps (a) and/or
(b) are disclosed in EP-A-0767486, EP-A-0993029, EP-A-1132952, U.S.
Pat. No. 6,391,219 and references cited herein. In an embodiment of
the present invention, cleaved surfaces may be smoothened before
further processing, as described for instance in U.S. Pat. No.
6,391,219. In a preferred embodiment of the invention, the original
substrate can be re-used directly and/or be prepared for
re-use.
[0026] In accordance with the present invention, the fabrication of
a device, or at least part thereof, takes place on a free-standing
thin film, before transfer of film and device to a final substrate
which becomes part of the final thin film device, preferably of
low-cost but highly efficient.
[0027] Throughout steps (b) to (d), in the present context, the
thin film is often referred to as a free-standing film, also after
transfer to a dummy substrate in step (c). The term "free-standing"
refers to the fact that the film is not attached or physically
bonded to the dummy substrate but merely placed or positioned on
it, possibly fixed in between two supports to give mechanical
strength to the porous semiconductor layer.
[0028] The fabricated device to be transferred to the foreign
substrate can be a non-finished or intermediate device. Some
processing steps to for instance finish the device, in other words
to achieve a properly working device, can be performed after
bonding of the intermediate device on a foreign substrate.
According to aspects of the present invention, at least those
processing steps are performed on the free-standing device placed
on (but not physically attached to) a dummy support that would
otherwise be limited with respect to temperature and/or other
process parameters. In a preferred embodiment of the invention, at
least an epitaxial active semiconductor layer is fabricated on the
free-standing thin device before transfer and attachment to a
foreign substrate on which it is then finished.
[0029] According to a preferred embodiment of the present invention
the steps (c) and (d) are performed twice, such that the repeated
step (c) unveils the unprocessed side of the thin film after the
first (d) step. In other words steps (c) and (d) can be repeated
such that (d) is performed ones on one side of the thin film and
ones on the other side of the thin film.
[0030] In an advantageous embodiment, the step of the fabrication
of a device comprises the deposition of an active semiconductor
layer on said thin film. In another embodiment, this formation of a
semiconductor layer is further followed by the fabrication of a
device, which can be contacted, on said active semiconductor layer.
The device may be any suitable device known in the art.
[0031] The lift-off process according to the present invention can
be achieved by immersing the substrate in a HF solution in
concentration between 12 and 35% and using current densities
between 50 and 250 mA/cm.sup.2 without changing any other
parameters.
[0032] Preferably, the deposition of the active layer is performed
by epitaxial Chemical Vapor Deposition (CVD).
[0033] The porous semiconductor layer can be a crystalline or
amorphous semiconductor material including silicone germanium,
III-V materials such as GaAs, InGaAs and semiconducting polymers,
for example.
[0034] The foreign substrate can be any substrate. For example, the
foreign substrate may comprise a low-cost substrate such as glass
or a polymeric material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 represents two manufacturing sequences of the prior
art for the fabrication of solar cells (PSI process [A-B-C-E-G] and
ELTRAN.RTM. process [A-B-D-F-H]).
[0036] FIG. 2 represents the manufacturing sequence of the present
invention for the fabrication of a solar cell.
[0037] FIG. 3 illustrates the process of porous film production
where pore branching results in increased lateral porosity followed
by the film separation (lift-off process). This process is
described in detail in EP-A-1132952.
[0038] FIG. 4 is a picture of the free-standing thin film device
after peeling and before the transfer to the support. The transfer
can be done, e.g., manually as shown.
[0039] FIG. 5 represents the set-up used for the porous
semiconductor layer formation according to EP-A-1132952.
[0040] FIG. 6 is representing the dummy substrate to hold the
porous semiconductor film during the epitaxial layer deposition.
The porous film is not attached or physically bonded to the dummy
substrate.
[0041] FIG. 7 represents an adhesived device on a foreign
substrate.
DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS
[0042] The widely known silicon on insulator (SOI) technology
involves the formation of a monocrystalline semiconductor layer on
an insulating material, such as silicon oxide or glass.
[0043] A particular embodiment of the present invention is related
to the fabrication of solar cells which are described in details in
FIGS. 1 and 2.
[0044] The sequence of the prior art process is described in FIG. 1
while FIG. 2 represents the manufacturing process for the
fabrication of a solar cell according to the present invention.
[0045] FIG. 1 [A-B-C-E-G] represents, according to the prior art,
the preparation of a porous silicon layer (10, double layer 1+2) on
a substrate (3) (FIG. 1A) followed by an epitaxial silicon layer
(7) deposition (FIG. 1B), fabrication of the contacted device (6)
on the substrate (FIG. 1C) and finally the separation of the
device, including the epitaxial silicon layer (7) and a portion of
the porous silicon layer (10), from the substrate (3) (FIG. 1E) and
the transfer to a foreign substrate (8) (FIG. 1G) in order to
possibly re-use the original substrate (3).
[0046] FIG. 1 [A-B-D-F-H] represents a similar process according to
the prior art wherein the preparation of a porous silicon layer
(10, double layer 1+2) on a substrate (3) (FIG. 1A) is followed by
an epitaxial silicon layer (7) deposition (FIG. 1B), the device
(11) fabrication, which is not yet contacted, on said substrate,
the separation of said device (11) from the substrate (3) (FIG.
1D), and the transfer to a foreign substrate (8) (FIG. 1F) in order
to possibly re-use the original substrate (3). The contacts (6) of
the device (11) are then formed after the bonding on the foreign
substrate (8) (FIG. 1H).
[0047] FIG. 2 illustrates an exemplary method for manufacturing a
solar cell. The method comprises the formation of a porous
semiconductor layer (double layer 1+2, step I) in the form of a
thin-film (10) on an original substrate (3), the formation being
immediately followed by the separation of the thin-film (10) by a
lift-off process from the original substrate (3) (step J), transfer
of the thin film (10) to a dummy substrate or support (4, step K),
and the preparation of the device (11) including epitaxial silicon
layer (7) deposition and, in certain embodiments, contact (6)
fabrication (step L). Finally, the whole device (11) is transferred
to a foreign substrate (8) in order to realize the solar cell (step
M). In one embodiment, contact (6) fabrication can take place after
transfer of a non-finished device to the foreign substrate (8).
[0048] The preferred lift-off process involving the film separation
from its substrate is described in details in FIG. (3). Other
lift-off or peeling-off processes are well-known in the art.
[0049] The pore formation according to this preferred lift-off
process starts at a certain position, it goes straight down in the
semiconductors as shown in FIG. 3 (zone 31). When the pores are not
deep enough, the reaction occurs at the bottom of the pore. At this
time, there are sufficient fluoride ions available at the bottom
but certainly less than the number of fluoride ions available at
the surface since they have to diffuse through the pore to the
point of reaction. Porosity of the layer increases as in the
concentration of HF in solution decreases. Although the initial
F-containing solution is not replaced, an in-situ change of
concentration is obtained. Therefore, as we go deeper, porosity of
the layer increases. The porosity gradient occurs from the point
where the availability of the fluoride ion is affected by the
diffusion through the pores.
[0050] As pores go sufficiently deep in silicon, the fluoride ion
concentration at the point of reaction reduces to a very low level
as compared to the surface concentration. This results in the shift
of the point of reaction to a slightly higher level because of very
high resistance of the lowest part of the pore. This shift in the
reaction gives rise to the formation of the branches of the pores
(zone 32). For every dissolution of a silicon atom, one hydrogen
molecule results as a product of the electrochemical etching. The
hydrogen molecules exert force on the walls of the pores. At some
points, because of the branching of pores, the walls become very
thin and not able to withstand the hydrodynamic pressure exerted by
the hydrogen molecules. This results in horizontal cracks in the
layer. The presence of sufficient horizontal cracks results in the
separation of the layer from the substrate (zone 33).
[0051] An example of a free-standing thin film is described in FIG.
4 after the peeling-off and before the transfer to a support. The
transfer can be performed manually, as shown. Alternatively the
transfer can be performed with a device that holds the thin film by
suction.
[0052] The systems and methods described herein increase
possibilities in the formation of porous layers in form of thin
films by proposing a new sequence for the preparation of free
standing thin-film devices to be used for SOI structures, such as
solar cells, for example. Examples of such porous semiconductor
layers called hereunder as PSL could be crystalline and amorphous
semiconductor material including silicon, germanium, III-V
materials such as Ga As, InGaAs and semiconducting polymers.
[0053] In conventional techniques a double porosity layer is
prepared by electrochemical etching and by changing one of the
formation parameters such as the electrolytic current density or
the HF concentration of the electrolytic solution.
[0054] The parameters of the porous silicon formation can also
remain unchanged such that the separation is reached by allowing
the current to flow for a sufficient amount of time (see
EP-A-1132952).
[0055] An example of a preferred set-up used for the formation of
the double porous semiconductor layers is described below with
reference to FIG. 5.
[0056] FIG. 5 illustrates an exemplary set-up used for the
formation of the porous semiconductor layer (PSL). In FIG. 5, the
platinum electrode (55), which is resistant against hydrofluoric
acid, acts as a negative electrode. The bottom plate (53) (e.g.,
stainless steel plate), which is in contact with the silicon wafer
(3) (polished side up), acts as a cathode. The rubber ring (52)
prevents the outflow of the solution from the contact area of the
Teflon.RTM. beaker (54) and wafer substrate (3). The rubber ring is
kept under pressure by the beaker (54), which in turn is
pressurized by a stainless steel threaded ring (not shown). In one
embodiment the setup also comprises an etching solution (51).
[0057] According to the process of the present invention, after the
preparation of the porous silicon layer, the film obtained is
immediately separated from the substrate by one or more processes
and then the fabrication of the device for the solar cell is
carried out without permanently transferring the film on other
substrates. This makes the process very simple because under these
conditions, no worry is justified about the foreign substrate as
well as original substrate and glues on which the film has to be
transferred. At the same time this gives a full freedom of process
parameters. Since there is no limitation due to the foreign
substrate and due to porosity of the semiconductor layer, any
temperature cycle can be used.
[0058] In an advantageous embodiment, full control of the process
of separating the porous silicon layer is possible,) avoiding
potential nuisances known to occur with certain prior art methods.
For instance, if the porosity of the high porosity layer (2) is not
high enough, separation after epitaxial deposition on the thin film
in accordance with such prior art method may not be possible so
that the lift-off process becomes unsuccessful. If the porosity of
the high porosity layer is too high then it may separate unwantedly
during the epitaxial layer deposition In an advantageous
embodiment, the formation and lift-off of porous layers (steps (a)
and (b)) are similar to the method described in EP-A-1132952 and
circumvents the above-described problems.
[0059] With the method according to the present invention there is
also no limitation on the epitaxial layer deposition temperature
which in prior art cases can be limited due to thermal stability of
the high-porosity layer and/or on the thermal stability of the
substrate and glue.
[0060] Since the film is not physically attached to any substrate
during epitaxy and cell fabrication, there are no concerns about
the physical and chemical properties of any foreign substrate
and/or glue and the impact of processing steps on these
properties.
[0061] It has been shown that the growth of an epitaxial layer (7)
on a porous thin film, which is attached, for instance glued on a
substrate (e.g., dummy substrate), is not obvious, due to the
impact on the glue of, for instance, the high temperatures used.
(typically 1050.degree. C.).
[0062] Both sides of the epitaxial layer are available during the
complete cell fabrication process so there is more freedom of cell
design and processing.
[0063] Preferably the fabrication of the device itself is performed
by transferring the lift-off film (10) to a dummy support (4) in
order to realize the epitaxial deposition (FIG. 6). The film can be
clamped by for instance clamps (61). The dummy substrate can be any
substrate that provides the necessary support during device
fabrication and/or allows freedom of process parameters and/or does
not interact with the thin film in a way that the latter would
become bonded to the dummy support during said device fabrication.
It is preferably resisting high temperatures (e.g., 1050.degree.
C.). It is preferably inert to process steps which are performed
during device fabrication while positioned on the dummy substrate.
In a preferred embodiment of the invention the dummy substrate is a
silicon substrate or a quartz substrate In a preferred embodiment
according to the invention, the dummy support may be re-used.
[0064] The present invention discloses a very attractive method for
the preparation of low-cost, high quality structures, such as SOI
structures or solar cells. The method according to the present
invention clearly has many advantages over methods presently known
in the art.
[0065] FIG. 7 illustrates an exemplary final structure, such as a
solar cell, whereon the active device (11) has been attached by an
adhesive (71) on a low-cost substrate (8).
[0066] According to a preferred embodiment related to the solar
cell manufacturing process, the processing is more simple and
provides a total freedom in terms of processing parameters,
compared to the conventional techniques, e.g., using porous silicon
sacrificial layers for thin film transfer processes. The transfer
process according to the present invention occurs directly after
the preparation of the porous silicon film on a dummy substrate
which makes an intermediate layer as in the prior art useless. Such
intermediate layers (e.g., hydrogen silsequioxane) require high
temperature stability (1100.degree. C.) and matching thermal
expansion coefficients as well as low impurity contents. For the
substrate, similar requirements are necessary. Contrary thereto,
for the process of the present invention, no transfer to the real
substrate is performed and therefore no intermediate layer is used.
The handling of the thin layer (film) is the biggest challenge but
this difficulty is compensated by fewer constraints due to the
reduced number of layers and more freedom in terms of process
parameters. The monocrystalline Si thin film solar cells can be
attached to any substrate (even flexible substrates) and the
handling of this film remains the most critical point.
[0067] Aspects of the present invention include a high quality
method which allows production of a thin semiconductor film
comprising a porous layer and a high quality epitaxial layer on top
of said porous layer, and transfer it onto a foreign substrate. The
resulting device can be used in a number of applications including,
but not limited to, the following: [0068] terrestrial solar cells,
due to their low-cost, [0069] space solar cells due to their light
weight (thin-film) combined with high efficiency, [0070] SOI
structures due to their high quality of the epitaxial layers.
[0071] The following example is offered by way of illustration, not
by way of limitation.
EXAMPLES
Example 1
Fabrication of Solar Cells
[0072] A 20 .mu.m porous silicon film is separated from highly
doped P-type, <100> silicon by electrochemical etching in an
electrolyte bath containing HF. After annealing of a porous silicon
film at 1050.degree. C. in H.sub.2, P-type silicon layer with 20
.mu.m thickness is deposited using conventional Chemical Vapor
Deposition (CVD). In the first trial, a simple two side-contacted
solar cell without any photolithography is applied for such
free-standing film. An efficiency of 10.6% is achieved for a small
area cell (1 cm.sup.2). The other cell parameters are as follows:
V.sub.oc--581.3 mV, I.sub.sc--30.29 mA/cm.sup.2 and FF--60.1%.
Internal Quantum Efficiency (IQE) analysis reveals that the
spectral response of free-standing film with Al backside
metallization is significantly increased in the infrared wavelength
region as compared to the cell transferred to conventional ceramic
substrates.
[0073] The process of the present invention comprises the following
four steps of fabrication:
[0074] a) Porous Silicon Formation and Separation From the Reusable
(Original) Substrate. (See Also EP-A-1132952)
[0075] For all experiments highly boron doped p-type
mono-crystalline CZ-Si substrates with a resistivity in the range
of 0.02-0.05 .OMEGA.-cm and an area of 5.times.5 cm.sup.2 are used.
Porous silicon formation is carried out in a conventional PTFE
(Teflon) cell with a silicon sample as anode and a platinum counter
electrode as shown in FIG. 5. The electrolyte contained of HF and
acetic acid is used. Porous silicon formation is carried out at
current density ranging from 50 to 250 mA/cm.sup.2 and HF
concentration ranging from 12 to 35 vol % at room temperature under
background illumination.
[0076] b) Transfer of the Porous Semiconductor Layer to a Dummy
Substrate.
[0077] The porous semiconductor layer is not attached or physically
bonded to the dummy substrate but fixed in between two supports to
give mechanical strength to the porous semiconductor layer.
Therefore we call it `free-standing film` or `free-standing solar
cell`.
[0078] c) Device Fabrication.
[0079] E.g., epitaxial silicon layer deposition: deposition of
active layer can be carried out with and without pre-annealing of
porous layer in H.sub.2 ambient at 1050.degree. C. for 30 min. In
the first case, the aim is to convert the porous silicon into quasi
monocrystalline silicone (QMS) which provides a good seeding layer
for a CVD layer deposition. An active layer of 10-30 microns is
deposited using dichlorosilane (DCS) or trichlorosilane (TCS) at
1050.degree. C. and 1130.degree. C. respectively. The porous
silicon film is kept (no permanent bonding) between two silicon
substrates, with the window on the top substrate for CVD
deposition.
[0080] One side contacted and two side contacted solar cells are
fabricated while keeping the Porous-Silicon+Epi essentially
free-standing (no permanent bonding to any substrate). Efficiency
of 10.6% has been achieved on 1 cm.sup.2 area.
[0081] d) Attach or Transfer Solar Cell on Foreign Substrate.
[0082] In the final step of fabrication solar cell is transferred
to a foreign substrate like Glass, Plastic, etc., using some
adhesive as for instance glue.
* * * * *