U.S. patent application number 13/533229 was filed with the patent office on 2013-11-28 for removal of stressor layer from a spalled layer and method of making a bifacial solar cell using the same.
This patent application is currently assigned to INTERNATIONAL BUSINESS MACHINES CORPORATION. The applicant listed for this patent is Stephen W. Bedell, Keith E. Fogel, Bahman Hekmatshoartabari, Paul A. Lauro, Ning Li, Devendra K. Sadana, Ghavam G. Shahidi, Davood Shahrjerdi. Invention is credited to Stephen W. Bedell, Keith E. Fogel, Bahman Hekmatshoartabari, Paul A. Lauro, Ning Li, Devendra K. Sadana, Ghavam G. Shahidi, Davood Shahrjerdi.
Application Number | 20130312819 13/533229 |
Document ID | / |
Family ID | 49620638 |
Filed Date | 2013-11-28 |
United States Patent
Application |
20130312819 |
Kind Code |
A1 |
Bedell; Stephen W. ; et
al. |
November 28, 2013 |
REMOVAL OF STRESSOR LAYER FROM A SPALLED LAYER AND METHOD OF MAKING
A BIFACIAL SOLAR CELL USING THE SAME
Abstract
A stressor layer used in a controlled spalling method is removed
through the use of a cleave layer that can be fractured or
dissolved. The cleave layer is formed between a host semiconductor
substrate and the metal stressor layer. A controlled spalling
process separates a relatively thin residual host substrate layer
from the host substrate. Following attachment of a handle substrate
to the residual substrate layer or other layers subsequently formed
thereon, the cleave layer is dissolved or otherwise compromised to
facilitate removal of the stressor layer. Such removal allows the
fabrication of a bifacial solar cell.
Inventors: |
Bedell; Stephen W.;
(Wappingers Falls, NY) ; Fogel; Keith E.;
(Hopewell Junction, NY) ; Hekmatshoartabari; Bahman;
(White Plains, NY) ; Lauro; Paul A.; (Brewster,
NY) ; Li; Ning; (White Plains, NY) ; Sadana;
Devendra K.; (Pleasantville, NY) ; Shahidi; Ghavam
G.; (Round Ridge, NY) ; Shahrjerdi; Davood;
(White Plains, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Bedell; Stephen W.
Fogel; Keith E.
Hekmatshoartabari; Bahman
Lauro; Paul A.
Li; Ning
Sadana; Devendra K.
Shahidi; Ghavam G.
Shahrjerdi; Davood |
Wappingers Falls
Hopewell Junction
White Plains
Brewster
White Plains
Pleasantville
Round Ridge
White Plains |
NY
NY
NY
NY
NY
NY
NY
NY |
US
US
US
US
US
US
US
US |
|
|
Assignee: |
INTERNATIONAL BUSINESS MACHINES
CORPORATION
Armonk
NY
|
Family ID: |
49620638 |
Appl. No.: |
13/533229 |
Filed: |
June 26, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13481795 |
May 26, 2012 |
|
|
|
13533229 |
|
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|
Current U.S.
Class: |
136/256 ;
257/618; 257/E29.005 |
Current CPC
Class: |
H01L 21/02002 20130101;
Y02E 10/547 20130101; H01L 31/1896 20130101; H01L 31/0684
20130101 |
Class at
Publication: |
136/256 ;
257/618; 257/E29.005 |
International
Class: |
H01L 31/02 20060101
H01L031/02; H01L 29/06 20060101 H01L029/06 |
Claims
1. A flexible, bifacial solar cell comprising: a semiconductor
substrate having a thickness less than fifty microns; an emitter
junction formed on a first side of the semiconductor substrate; a
back junction formed on a second side of the semiconductor
substrate; a transparent conductive layer electrically associated
with each junction, a metal contact layer electrically associated
with each transparent conductive layer, and a transparent handle
substrate adjoining one of the metal contact layers.
2. The flexible, bifacial solar cell of claim 1, wherein the
semiconductor substrate comprises silicon.
3. A structure comprising: a semiconductor substrate; a flexible
handle substrate on a first side of the semiconductor substrate; a
second handle substrate on a second side of the semiconductor
substrate; a stressor layer between the semiconductor substrate and
the flexible handle substrate, and a cleave layer positioned
between the stressor layer and the semiconductor substrate, the
cleave layer being selectively etchable or dissolvable with respect
to the semiconductor substrate and stressor layer.
4. The structure of claim 3, wherein the cleave layer has a lower
fracture toughness value (K.sub.lc) than that of the material
comprising the semiconductor substrate.
5. The structure of claim 3, wherein the stressor layer is a metal
layer.
6. The structure of claim 5, further including a first junction
layer on the first side of the semiconductor substrate and between
the semiconductor substrate and the cleave layer.
7. The structure of claim 6, further including a second junction
layer on the second side of the semiconductor substrate.
8. The structure of claim 7, wherein the second handle substrate is
transparent.
9. The structure of claim 8, wherein the semiconductor substrate
has a thickness of less than fifty microns.
10. The structure of claim 3, wherein the second handle substrate
is transparent.
11. The structure of claim 10, further including an emitter
junction on the semiconductor substrate.
12. The structure of claim 3, wherein the cleave layer is laterally
recessed with respect to the semiconductor substrate and the
stressor layer, the cleave layer being positioned such that a force
exerted on the cleave layer through the stressor layer causes
spalling through the cleave layer.
13. The structure of claim 12, wherein the stressor layer is a
metal layer.
14. The structure of claim 13, wherein the semiconductor substrate
comprises silicon.
15. The structure of claim 14, further including an emitter
junction on the semiconductor substrate.
16. The structure of claim 15, wherein the second handle substrate
is transparent.
17. The structure of claim 16, wherein the semiconductor substrate
has a thickness of less than fifty microns.
18. The structure of claim 17, further including a back junction on
the semiconductor substrate.
19. The structure of claim 12, further including emitter and back
junctions on the semiconductor substrate, and wherein the
semiconductor substrate has a thickness of less than fifty microns
and the second handle substrate is transparent.
20. The structure of claim 19, wherein the semiconductor substrate
comprises silicon.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/481,795 filed May 26, 2012, the complete
disclosure of which is expressly incorporated herein by reference
in its entirety for all purposes.
FIELD
[0002] The present disclosure relates to the physical sciences and,
more particularly, to controlled spalling techniques and devices
such as bifacial solar cells that can be fabricated using such
techniques.
BACKGROUND
[0003] Controlled spalling technology can be employed to remove
thin substrate (e.g. silicon) film from a relatively thick
substrate, enabling a wide range of applications including low cost
solar cells and thin film flexible electronic devices and
structures. The metal stressor layer used in spalling procedures
also acts as a supporting layer to assist with handling of the
spalled thin film, making thin semiconductor film handling much
easier and more reliable. It is necessary, however, to remove the
metal stressor layer in some applications, for example to obtain
free standing semiconductor thin film to be compatible with high
temperature device processing or to expose the original
semiconductor substrate surface which is in contact with the metal
stressor layer after spalling. Attempts have been made to remove
the metal stressor layer using wet and/or dry chemical etching.
Non-uniformity in etch rate while employing wet chemical etching
through a thick nickel stressor layer may induce cracks in the
spalled film. Additionally, complete removal of the thick nickel
layer using wet/dry etching may be tune consuming, lowering
throughput.
BRIEF SUMMARY
[0004] In accordance with the principles discussed herein,
fabrication methods are disclosed for fabricating semiconductor
devices such as photovoltaic structures.
[0005] A method provided herein comprises obtaining a first
structure including a host substrate comprising a semiconductor
material, a flexible handle substrate, a stressor layer, and a
cleave layer, the cleave layer being positioned between the host
substrate and the stressor layer, and the stressor layer being
positioned between the cleave layer and the flexible handle
substrate. The method further includes separating a portion of the
semiconductor from the host substrate via controlled spalling. A
second structure comprising the flexible handle substrate, the
stressor layer, the cleave layer and a residual substrate layer are
spalled from the substrate. The method further includes
compromising the cleave layer and removing the stressor layer from
the second structure.
[0006] A further method comprises obtaining a first structure
including a host substrate comprising a semiconductor material, a
first junction on a first side of the host substrate, a cleave
layer positioned over the first junction, a stressor layer over the
cleave layer, and a flexible handle substrate, the stressor layer
being positioned between the cleave layer and the flexible handle
substrate. The method further includes spalling the host substrate
to separate a residual substrate layer from the host substrate,
forming at least one of a second junction and a metal contact on a
side of the residual substrate layer opposite from the first
junction, attaching a second handle substrate on a side of the
residual substrate layer opposite from the flexible handle
substrate, compromising the cleave layer, and removing the stressor
layer.
[0007] A further method comprises obtaining a first structure
comprising: a substrate layer comprised of a semiconductor
material, a flexible handle substrate on a first side of the
substrate layer, a stressor layer between the substrate layer and
the flexible handle substrate, a cleave layer between the substrate
layer and the stressor layer, and a second handle substrate on a
second side of the substrate layer opposite from the flexible
handle substrate. The method further includes compromising the
cleave layer and removing the stressor layer.
[0008] A further method provided herein includes fabricating a
first structure by: forming a cleave layer over a semiconductor
host substrate, forming a metal stressor layer over the cleave
layer, and forming a flexible handle substrate over the metal
stressor layer. The method further includes spalling through the
host substrate of the first structure, thereby forming a second
structure including the flexible handle substrate, the metal
stressor layer, the cleave layer, and a residual substrate layer
from the host substrate. The method further includes attaching a
second handle substrate to the second structure on a side of the
residual substrate layer opposite from the metal stressor
layer.
[0009] An exemplary flexible bifacial solar cell structure in
accordance with certain aspects of the disclosure includes a
semiconductor substrate having a thickness less than fifty microns,
an emitter junction formed on a first side of the semiconductor
substrate, a back junction formed on a second side of the
semiconductor substrate, a transparent conductive layer
electrically associated with each junction, a metal contact layer
electrically associated with each transparent conductive layer, and
a transparent handle substrate adjoining one of the metal contact
layers.
[0010] A second exemplary structure comprises a semiconductor
substrate, a flexible handle substrate on a first side of the
semiconductor substrate, a second handle substrate on a second side
of the semiconductor substrate, a stressor layer between the
semiconductor substrate and the flexible handle substrate, and an
etchable or dissolvable cleave layer positioned between the
stressor layer and the semiconductor substrate. Further embodiments
of the second exemplary structure include one or more additional
features, such as the cleave layer having a lower fracture
toughness value (K) than that of the material comprising the
semiconductor substrate, emitter and/or back junctions on the
semiconductor substrate, a metal stressor layer, a transparent
second handle substrate, a semiconductor substrate thickness of
less than fifty microns, and a laterally etched cleave layer.
[0011] As used herein, "facilitating" an action includes performing
the action, making the action easier, helping to carry the action
out, or causing the action to be performed. Thus, by way of example
and not limitation, instructions executing on one processor might
facilitate an action carried out by instructions executing on a
remote processor, by sending appropriate data or commands to cause
or aid the action to be performed. For the avoidance of doubt,
where an actor facilitates an action by other than performing the
action, the action is nevertheless performed by some entity or
combination of entities.
[0012] Substantial beneficial technical effects are provided by the
exemplary structures and methods disclosed herein. For example, one
or more embodiments may provide one or more of the following
advantages:
[0013] Simplifying the post-spalling device fabrication
process;
[0014] Mechanical flexibility of fabricated device;
[0015] Possible reuse of host semiconductor substrate;
[0016] Bifacial solar cell operation.
[0017] These and other features and advantages of the disclosed
methods and structures will become apparent from the following
detailed description of illustrative embodiments thereof, which is
to be read in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIGS. 1A-F show a flow diagram of a fabrication method in
accordance with a first exemplary embodiment;
[0019] FIGS. 2A-G show a flow diagram of a method of fabricating a
photovoltaic structure in accordance with a second exemplary
embodiment;
[0020] FIG. 3 shows alternative flow diagrams of alternative
fabrication steps for making a photovoltaic structure;
[0021] FIG. 4 is an I-V chart showing test results for the
structures obtained using the alternative fabrication steps of FIG.
3, and
[0022] FIGS. 5A-C show a flow diagram of a fabrication method in
accordance with a third exemplary embodiment.
DETAILED DESCRIPTION
[0023] Methods of removing a stressor layer from spalled material
using a cleave layer are disclosed. The fabrication of bifacial
solar cells using such methods is further disclosed.
[0024] A method in accordance with a first exemplary embodiment is
comprised of the following steps (i) growing or depositing a
material (hereafter referred to as a cleave layer) comprised of a
material (e.g. ZnO:Al (AZO), indium tin oxide (ITO), silicon
nitride (SiN) or silicon dioxide (SiO2)) having a lower fracture
toughness value (K.sub.lc) than that of the host substrate material
(in this exemplary embodiment Si) (ii) partial lateral etching of
the cleave layer, and (iii) spalling through the cleave layer to
detach the stressor layer and the handle layer used for spalling.
The stressor layer is preferably though not necessarily a metal
layer such as nickel or titanium. Such a method is illustrated
schematically in FIG. 1.
[0025] Referring to FIG. 1A, first structure 10 is formed by (i)
growing or depositing the cleave layer 14 on the host substrate 12,
(ii) growing or depositing a metal stressor layer 16 on top of the
cleave layer, and (iii) bonding the substrate to a flexible handle
layer 18 on the metal stressor layer side. Referring to FIG. 1B, a
thin layer 12A of the host substrate material 12 is spalled using a
controlled spalling process, such as disclosed in U.S. Pub. No.
2010/0307572, which is incorporated by reference herein. The
characteristics of the metal stressor layer 16 such as thickness
and stress and the flexible handle layer 18 are adjusted to create
a fracture 20 in the host substrate 12 that ultimately separates
the flexible handle layer, metal stressor layer, cleave layer and
the thin residual layer 12A from the host substrate. The spalled
substrate layer 12A is then bonded to a second handle substrate 24,
which may or may not be flexible, on the spalled side. The second
handle substrate 24 has sufficient thickness to facilitate handling
of the structure to which it is affixed during and/or following
device fabrication. As shown in FIG. 1C, the resulting structure 30
includes the flexible handle layer 18, the metal stressor layer 16,
the cleave layer 14, the residual layer 12A, a bonding layer 22,
and the second handle substrate 24. It will be appreciated that one
or more additional thin film layers (not shown) may be formed on
the thin residual layer 12A prior to bonding the second handle
substrate. It will further be appreciated that the host substrate
12 may either be monolithic or include a plurality of layers.
[0026] Referring to FIG. 1D, partial lateral etching of the cleave
layer 14 is effected by exposing the structure 30 to a chemical
etch including but not limited to a hydrogen fluoride (HF) or
hydrogen chloride (HCL) containing solution. Applying shear stress
using the flexible handle substrate 18, the stressor layer 16 and
the flexible handle substrate 18 are detached by forming a fracture
26 in and spalling through the cleave layer 14 as shown in FIG. 1E.
(The terms flexible handle layer and flexible handle substrate are
used interchangeably.) Applying shear stress to the handle
substrate to detach the flexible handle substrate and the stressor
is typically performed outside the etch solution; however, it is
possible to perform this step inside the etch solution as well.
Residual cleave layer material remaining on the residual (e.g.
silicon) layer(s) is removed in this exemplary embodiment using the
same solution employed to effect partial lateral etching of the
cleave layer 14. A different etch solution may be used as well, if
desired.
[0027] Methods as disclosed herein are applicable to the
fabrication of thin film devices including photovoltaic devices.
The methods are amenable to wafer scale applications. As known in
the art, a photovoltaic device (solar cell) is comprised of at
least one junction, i.e. the emitter (front) junction. The emitter
junction is comprised of at least one semiconductor material having
the opposite conductivity as that of the substrate. The emitter may
be formed using various techniques known in the art, such as
diffusion, implantation, deposition/growth of the emitter layer(s),
and combinations thereof. A photovoltaic device may also
optionally, but preferably, include a second junction, on the
second (back) side of the substrate, comprised of at least one
semiconductor material having the same conductivity type as that of
the substrate. Similarly, the back junction may be formed by
various techniques known in the art, such as diffusion,
implantation, growth/deposition of layers, and combinations
thereof. Fabrication of a solar cell device according to the
methods disclosed herein offers the advantage of reusing the
substrate material, mechanical flexibility of the solar cell,
bifacial operation of the solar cell, or combinations thereof.
[0028] Solar cell fabrication according to an exemplary embodiment
of the disclosed methods includes the following steps: (i) forming
a first junction on the a first side of the substrate, (ii) forming
a cleave layer above the first junction, (iii) forming a metal
stressor layer above the cleave layer, (iv) bonding a flexible
handle substrate on the metal stressor layer side of the substrate,
(v) spalling the substrate using the flexible handle substrate,
forming a residual substrate layer, (vi) forming a second junction
(or solely a metal contact) on a second (spoiled) side of the
substrate, (vii) bonding the substrate onto a second handle
substrate (which may or may not be flexible) on the second
(spalled) side of the substrate, (viii) compromising the cleave
layer as disclosed herein (e.g. by partial lateral etching or
dissolution of the cleave layer), and (ix) removing the first
handle substrate and the stressor layer from the first side of the
substrate (e.g. by spalling through the cleave layer if partially
compromised or simple displacement of the stressor metal layer with
respect to the residual substrate layer if the cleave layer is
entirely compromised. Depending on the type of solar cell
technology being used, further processing steps as known in the
art, may follow or be included in between these steps as necessary.
If the second handle substrate is optically transparent, the solar
cell may be operated in bifacial mode.
[0029] An exemplary process in illustrated schematically for a Si
heterojunction solar cell comprised of front and back PECVD stacks
in FIGS. 2A-G. The order of the formation of the front and back
PECVD stacks may be reversed. Materials other than Al-doped
zinc-oxide (ZnO:Al) may be used to form the cleave layer. Examples
include but are not limited to indium-tin-oxide (ITO), silicon
nitride and silicon dioxide. In the case of ZnO:Al and ITO, the
lateral etch of the cleave layer may be performed in an HF
containing or an HCl containing solution. An HF containing solution
may be used in the case of nitride or oxide cleave layers as well.
The residual cleave layer after stressor layer removal may be
removed using the same etch solution if desired, prior to
subsequent processing steps. The exemplary process may be performed
by dissolving the cleave layer rather than lateral etching followed
by spalling, as described below with reference to FIG. 5.
[0030] Referring to FIG. 2A, a first structure 50 includes a host
substrate, which is a thick silicon wafer in this exemplary
embodiment. An emitter stack 54 is formed on the host substrate by
plasma-enhanced chemical vapor deposition (PECVD) and comprises a
first junction. A cleave layer 56 is formed over the emitter stack.
As discussed above, the cleave layer 56 has a lower fracture
toughness value (K.sub.lc) than that of the host substrate material
in embodiments where the cleave layer is to be spalled. A stressor
metal layer 58 is formed over the cleave layer. The stressor metal
layer may comprise nickel and be about 5-6 microns in thickness in
some embodiments. A flexible handle substrate 62 is attached to the
metal stressor layer side of the substrate 52 as shown in FIG. 2B.
The flexible handle substrate comprises a polyimide material in
some embodiments, such as Kapton tape. The flexible handle
substrate is employed as a mechanical handle for controlled
spalling of the host substrate 52. The spalling process results in
the separation of the stacked structure 60 shown in FIG. 2B from
the host substrate. The residual silicon layer 52A in the stacked
structure 60 has a thickness of fifty microns or less in some
embodiments, and is preferred for forming a flexible, bifacial
solar cell.
[0031] As shown in FIG. 2C, the structure 60 is subjected to an
etching solution to compromise the integrity of the cleave layer in
this exemplary embodiment. The cleave layer is laterally etched,
thereby weakening it, and facilitating crack initiation in the
cleave layer.
[0032] A back stack 72 is formed on the residual silicon layer 52A
by PECVD in this exemplary embodiment, and comprises a second
junction. (As discussed above, the first and second junctions can
be formed in either order.) A transparent conductive layer 56' is
formed on the back stack 72. The transparent conductive layer may
be a transparent conductive oxide such as ZnO:Al, like the cleave
layer 56. It will be appreciated that the transparent conductive
layer and the cleave layers may be comprised of the same or
different materials. They may accordingly have the same or
different etch characteristics. A metal layer comprising metal
fingers 74 is formed on the transparent conductive layer to
complete the structure 70 shown in FIG. 2D. It will be appreciated
that the etching step may be performed either prior to or after
formation of the transparent conductive layer. If the cleave layer
and transparent conductive layer are both comprised of etchable
materials, they will both be etched laterally. However, since the
cleave layer 56 is closest to the flexible handle substrate 62, the
spalling would still occur through the cleave layer as described
below with reference to FIG. 2F.
[0033] A second handle substrate 62' is bonded to the structure 70
to form the structure 80 shown in FIG. 2E. The second handle
substrate is attached to the side opposite the residual silicon
layer 52A from the flexible handle substrate. As discussed above,
the second handle substrate 62' may or may not be flexible. It can
be comprised of plastic, glass, metal or other materials that
facilitate handling of the structure 80 and other subsequently
formed structures. The flexible handle substrate exerts a force on
the weakened cleave layer 56 through the metal stressor layer 58,
separating the flexible handle substrate and stressor metal layer
by spalling. This step results in the structure 90 shown in FIG.
2F. As discussed above, any residual cleave layer remaining on the
structure 90 may be removed by etching. A second transparent
conductive layer 56' is deposited on the emitter stack 54 followed
by formation of metal fingers 74 on this layer. The bifacial
heterojunction photovoltaic structure 100 shown in FIG. 2G is
thereby provided.
[0034] A proof-of-concept experiment is illustrated in FIG. 3. A
PECVD emitter stack 54 comprised of n.sup.+ doped Si layers was
deposited on commercially available p.sup.-/p.sup.+ Si substrates
52. The substrate 52 was then divided into two pieces. The first
piece (control sample) was processed to form the ZnO:Al doped
electrode 56', the top metal grid 74 (fingers) and a bottom metal
contact 152, forming the photovoltaic structure 160. On the second
piece (test sample), a ZnO:Al layer was deposited as a cleave layer
56, followed by the deposition of the stressor metal layer 58. The
stressor metal layer was then removed by lateral etching in dilute
HF solution followed by spalling through the ZnO:Al cleave layer 56
as disclosed herein. The residual ZnO:Al layer was subsequently
removed in the dilute HF solution. The ZnO:Al electrode 56', front
metal grid 74, and the back contact metal 152 were then formed on
this sample, forming a second photovoltaic structure 150. (The
electrode/metal grid/back metal deposition steps were performed
simultaneously on this test sample and the control sample described
above, to eliminate possible run-to-run process variations.) The
experimental light I-V characteristics of the test sample 150 and
the control sample 160 are nearly identical as shown in FIG. 4. It
is accordingly evident that the disclosed method does not
compromise the performance of the resulting solar cell device.
[0035] FIGS. 5A-5C schematically illustrate an alternative process
for separating the metal stressor layer 58 from a residual
substrate layer 52A following spalling. As shown in FIG. 5A, an
exemplary structure 200 includes a silicon host substrate 52, a
ZnO:Al cleave layer 56, a metal (e.g. titanium or nickel) stressor
layer 58, and a flexible tape 62. Mechanical strain induced via the
flexible tape 62 causes the structure 210 above a fracture formed
in the substrate 52 to be spalled from the substrate, as shown in
FIG. 5B. A second handle substrate 62' is attached to the structure
210. The cleave layer 56 is then dissolved in solution as shown in
FIG. 5C. This is in contrast to being laterally etched and then
spalled as shown in FIGS. 1 and 2. Once the adhesion of the cleave
layer 56 to the adjoining layers has been adequately compromised, a
structure 220 comprising the residual silicon layer 52A and tape
separates from the metal stressor layer 58. An emitter stack and/or
other thin film layers can then be formed on the residual silicon
layer 52A, which is supported by the second handle substrate 62'
(e.g. a tape). The principles of this process can be incorporated
within the processes of FIGS. 1 and 2. The cleave layer 56 in this
exemplary process should: i) be sufficiently rigid to transfer
stress from the stressor metal layer to the substrate 52, ii) have
strong adhesion to both the stressor metal layer and the host
substrate or other layer which it adjoins, and iii) be quickly
dissolvable in solution to effect quick release of the metal
stressor layer. As discussed above, zinc oxide can be dissolved in
an acid solution containing HCl or HF. It can alternatively be
dissolved in hot acetone. It will be appreciated that the term
"cleave" as used to describe the cleave layer 56 this exemplary
embodiment does not suggest that this layer will be split or
fractured rather than dissolved. Mechanical agitation (e.g.
ultrasonic) of the structure 210 may be employed to facilitate the
dissolution of the cleave layer.
[0036] Given the discussion thus far, a method is provided that
includes obtaining a first structure including a host substrate
comprising a semiconductor material, a flexible handle substrate, a
stressor layer, and a cleave layer, the cleave layer being
positioned between the host substrate and the stressor layer, the
stressor layer being positioned between the cleave layer and the
flexible handle substrate. Such a first structure is shown by the
exemplary structure 10 in FIG. 1A. The method further includes
separating a portion of the semiconductor material from the host
substrate using controlled spalling. FIG. 1B shows the formation of
a fracture 20 in the host substrate using the flexible handle
substrate 18 during part of a controlled spalling process. The host
substrate is spalled, thereby forming a second structure including
the flexible handle substrate, the stressor layer, the cleave
layer, and a residual substrate layer from the host substrate. Such
a structure is also shown in FIG. 1B above the fracture 20. The
method further includes compromising the cleave layer and removing
the stressor layer from the second structure. FIG. 1D shows the
compromising of the cleave layer by lateral etching, though as
discussed above it could be compromised by dissolution in this or
other embodiments. Removal of the stressor layer 16 through
spalling is shown in FIG. 1E. The flow diagrams of FIGS. 2A-2F and
5A-5C and accompanying discussion above are also germane to this
method.
[0037] A method in accordance with a further exemplary embodiment
comprises obtaining a first structure including a host substrate
comprising a semiconductor material, a first junction on a first
side of the host substrate, a cleave layer positioned over the
first junction, a stressor layer over the cleave layer, and a
flexible handle substrate, the stressor layer being positioned
between the cleave layer and the flexible handle substrate. FIG. 2A
shows such an exemplary structure prior to attaching the flexible
handle substrate, which is shown in FIG. 2B. The method further
includes spalling the host substrate to separate a residual
substrate layer from the host substrate. FIG. 2B shows the spalling
of the host substrate, as further described above. At least one of
a second junction and a metal contact is formed on a side of the
residual substrate layer opposite from the first junction, such as
the second junction 72 shown in FIG. 2D. The method further
includes attaching a second handle substrate on a side of the
residual substrate layer opposite from the flexible handle
substrate. FIG. 2E shows the attachment of a second handle
substrate 62' on a side of the residual substrate layer 52A
opposite from the flexible handle substrate 62. The method further
includes compromising the cleave layer and removing the stressor
layer. FIG. 2C shows the lateral etching of the cleave layer 56,
which is thereby compromised. Stressor layer removal is shown in
FIG. 2F.
[0038] A further exemplary method includes obtaining a first
structure comprising a substrate layer comprised of a semiconductor
material, a flexible handle substrate on a first side of the
substrate layer, a stressor layer between the substrate layer and
the flexible handle substrate, a cleave layer between the substrate
layer and the stressor layer, and a second handle substrate on a
second side of the substrate layer opposite from the flexible
handle substrate. FIGS. 1C and 2E both show such a structure.
Referring to FIG. 1C, a semiconductor substrate 12A for this
structure 30 is obtained through spalling of the host substrate 12.
A flexible handle substrate 18 is on a first side of the substrate
layer 12A and a stressor layer 16 is between the substrate layer
12A and flexible handle substrate 18. The cleave layer 14 is
between the substrate layer 12A and the stressor layer 16. The
method further includes compromising the cleave layer, which is
shown in FIG. 1D and removing the stressor layer. Stressor layer
removal in the embodiment of FIG. 1E is accomplished by spalling
through the cleave layer. As discussed with respect to FIGS. 5A-C,
the cleave layer may instead by compromised through dissolution
followed by removal of the stressor layer.
[0039] A further exemplary method includes fabricating a first
structure by: forming a cleave layer over a semiconductor host
substrate, forming a metal stressor layer over the cleave layer,
and forming a flexible handle substrate over the metal stressor
layer. A first structure 10 fabricated in accordance with the
method is shown in FIG. 1A. The method further includes spalling
through the host substrate of the first structure, thereby forming
a second structure including the flexible handle substrate, the
metal stressor layer, the cleave layer, and a residual substrate
layer from the host substrate. FIG. 1B shows the step of spalling
through the host substrate 12 to form a second structure including
the flexible handle substrate 18, the stressor layer 16, the cleave
layer 14 and a residual substrate layer 12A. A second handle
substrate is attached to the second structure on a side of the
residual substrate layer opposite from the metal stressor layer, as
exemplified by FIG. 1C which shows the attachment of a second
handle substrate 24. FIGS. 2A, 2B and 2E likewise show the
fabrication of such a first structure, followed by spalling of the
host substrate 52 to form a second structure 60 and the attachment
of a second handle substrate 62'.
[0040] A structure is provided in accordance with a further aspect
that comprises a semiconductor substrate, a flexible handle
substrate on a first side of the semiconductor substrate, and a
second handle substrate on a second side of the semiconductor
substrate. FIG. 1D shows one exemplary embodiment including a
semiconductor substrate (residual layer 12A), a flexible handle
substrate 18 and a second handle substrate 24. FIG. 2E shows a
second exemplary structure 80 including these elements. The
structure further includes a stressor layer between the
semiconductor substrate and the flexible handle substrate. An
etchable or dissolvable cleave layer is between the metal stressor
layer and the semiconductor substrate. The cleave layer is selected
to have a lower fracture toughness value (K.sub.lc) than that of
the material comprising the semiconductor substrate if it is to be
spalled rather than dissolved. Stressor layers and cleave layers
are shown, respectively, in the structures 30 and 80 illustrated in
FIGS. 1D and 2E. In one or more embodiments of the structure, a
first junction layer is provided on a side of the semiconductor
substrate, such as the emitter layer 54 shown in FIG. 2E. In one or
more further embodiments of the structure, a second junction layer
is provided on a second side of the semiconductor substrate, such
as the back junction layer 72 shown in FIG. 2E. In one or more
embodiments of the structure, the stressor layer is a metal layer.
The second handle substrate is transparent in one or more
embodiments of the structure. The cleave layer is laterally etched
in one or more embodiments of the exemplary structure, such as
shown in FIGS. 1D and 2E. In one or more embodiments of the
exemplary structure, the thickness of the semiconductor substrate
is less than fifty microns. The semiconductor substrate comprises
silicon in one or more embodiments.
[0041] A flexible, bifacial solar cell is further provided in
accordance with the present disclosure. Referring to FIG. 2G, the
solar cell 100 includes a semiconductor substrate having a
thickness less than fifty microns. The residual silicon layer 52A
shown in FIG. 2G has such a thickness in this exemplary embodiment,
which provides for device flexibility. A first junction such as an
emitter layer is formed on a first side of the semiconductor
substrate and a second (e.g. back) junction is formed on a second
side of the semiconductor substrate. A transparent conductive layer
is electrically associated with each junction and a metal contact
layer is electrically associated with each transparent conductive
layer. A transparent handle substrate such as the handle substrate
62' adjoins one of the metal contact layers. In one or more
embodiments of the solar cell, the semiconductor substrate
comprises silicon.
[0042] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the invention. As used herein, the singular forms "a", "an" and
"the" are intended to include the plural forms as well, unless the
context clearly indicates otherwise. It will be further understood
that the terms "comprises" and/or "comprising," when used in this
specification, specify the presence of stated features, integers,
steps, operations, elements, and/or components, but do not preclude
the presence or addition of one or more other features, integers,
steps, operations, elements, components, and/or groups thereof.
Terms such as "above" and "below" are generally employed to
indicate relative positions as opposed to relative elevations
unless otherwise indicated.
[0043] The corresponding structures, materials, acts, and
equivalents of all means or step plus function elements in the
claims below are intended to include any structure, material, or
act for performing the function in combination with other claimed
elements as specifically claimed. The description of the present
invention has been presented for purposes of illustration and
description, but is not intended to be exhaustive or limited to the
invention in the form disclosed. Many modifications and variations
will be apparent to those of ordinary skill in the art without
departing from the scope and spirit of the invention. The
embodiment was chosen and described in order to best explain the
principles of the invention and the practical application, and to
enable others of ordinary skill in the art to understand the
invention for various embodiments with various modifications as are
suited to the particular use contemplated.
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