U.S. patent application number 15/934409 was filed with the patent office on 2018-07-26 for off-axis epitaxial lift process.
The applicant listed for this patent is ALTA DEVICES, INC.. Invention is credited to Melissa ARCHER, Thomas GMITTER, Gang HE, Siew NEO.
Application Number | 20180209018 15/934409 |
Document ID | / |
Family ID | 47711718 |
Filed Date | 2018-07-26 |
United States Patent
Application |
20180209018 |
Kind Code |
A1 |
GMITTER; Thomas ; et
al. |
July 26, 2018 |
OFF-AXIS EPITAXIAL LIFT PROCESS
Abstract
Embodiments described herein provide processes for forming and
removing epitaxial films and materials from growth wafers by
epitaxial lift off (ELO) processes. In some embodiments, the growth
wafer has edge surfaces with an off-axis orientation which is
utilized during the ELO process. The off-axis orientation of the
edge surface provides an additional variable for controlling the
etch rate during the ELO process and therefore the etch front may
be modulated to prevent the formation of high stress points which
reduces or prevents stressing and cracking the epitaxial film
stack. In one embodiment, the growth wafer is rectangular and has
an edge surface with an off-axis orientation rotated by an angle
greater than 0.degree. and up to 90.degree. relative to an edge
orientation of <110> at 0.degree..
Inventors: |
GMITTER; Thomas; (Sunnyvale,
CA) ; HE; Gang; (Cupertino, CA) ; ARCHER;
Melissa; (San Jose, CA) ; NEO; Siew;
(Sunnyvale, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ALTA DEVICES, INC. |
Sunnyvale |
CA |
US |
|
|
Family ID: |
47711718 |
Appl. No.: |
15/934409 |
Filed: |
March 23, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15876001 |
Jan 19, 2018 |
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15934409 |
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13210138 |
Aug 15, 2011 |
9994936 |
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15876001 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C30B 29/42 20130101;
C22C 29/00 20130101; C30B 29/40 20130101; C30B 29/00 20130101; C30B
33/10 20130101; C30B 33/08 20130101; C30B 33/06 20130101 |
International
Class: |
C22C 29/00 20060101
C22C029/00; C30B 33/06 20060101 C30B033/06 |
Claims
1. A growth wafer, comprising: a single substrate having a
crystalline lattice structure, wherein: the single substrate has
multiple edges that are non-parallel and non-perpendicular to a
cleave plane, and the single substrate has a facial surface with a
<001> orientation, off by up to 12.degree..
2. The growth wafer of claim 1, wherein the edges being
non-parallel and non-perpendicular to the cleave plane corresponds
to the edges having an off-axis orientation that is rotated from
the cleave plane by an angle from 0.degree. to 90.degree..
3. The growth wafer of claim 1, wherein the edges being
non-parallel and non-perpendicular to the cleave plane corresponds
to the edges having an off-axis orientation that is rotated from
the cleave plane by an angle from 30.degree. to 60.degree..
4. The growth wafer of claim 1, wherein the edges being
non-parallel and non-perpendicular to the cleave plane corresponds
to the edges having an off-axis orientation that is rotated from
the cleave plane by a 45.degree. angle.
5. The growth wafer of claim 1, wherein the growth wafer has a
rectangular shape or square shape, and the edges correspond to the
sides of the rectangular shape or the square shape.
6. The growth wafer of claim 1, wherein the growth wafer is cut or
diced from a circular growth wafer.
7. The growth wafer of claim 6, wherein the circular growth wafer
has a flat cut or a notch that indicates the cleave plane.
8. The growth wafer of claim 1, wherein the cleave plane is a
<110> orientation.
9. The growth wafer of claim 1, wherein the single substrate
includes elements from Group III, Group IV, or Group V.
10. The growth wafer of claim 1, wherein the single substrate is
configured to have a sacrificial layer deposited on the single
substrate, and the sacrificial layer is configured to have an
epitaxial film stack formed on the sacrificial layer.
11. A sacrificial layer for an epitaxial lift off process,
comprising: a layer formed on a single substrate having a
crystalline lattice structure, wherein: the single substrate has
multiple edges that are non-parallel and non-perpendicular to a
cleave plane, and the single substrate has a facial surface with a
<001> orientation, off by up to 12.degree., wherein the layer
is configured to have an epitaxial film stack formed on the layer
and to be etched away to separate the epitaxial film stack from the
single substrate.
12. The sacrificial layer of claim 11, wherein the edges being
non-parallel and non-perpendicular to the cleave plane corresponds
to the edges having an off-axis orientation that is rotated from
the cleave plane by an angle from 0.degree. to 90.degree..
13. The sacrificial layer of claim 11, wherein the edges being
non-parallel and non-perpendicular to the cleave plane corresponds
to the edges having an off-axis orientation that is rotated from
the cleave plane by an angle from 30.degree. to 60.degree..
14. The sacrificial layer of claim 11, wherein the edges being
non-parallel and non-perpendicular to the cleave plane corresponds
to the edges having an off-axis orientation that is rotated from
the cleave plane by a 45.degree. angle.
15. The sacrificial layer of claim 11, wherein the growth wafer has
a rectangular shape or square shape, and the edges correspond to
the sides of the rectangular shape or the square shape.
16. The sacrificial layer of claim 11, wherein the growth wafer is
cut or diced from a circular growth wafer.
17. The sacrificial layer of claim 16, wherein the circular growth
wafer has a flat cut or a notch that indicates the cleave
plane.
18. The sacrificial layer of claim 11, wherein the cleave plane is
a <110> orientation.
19. The sacrificial layer of claim 11, wherein the single substrate
includes elements from Group III, Group IV, or Group V.
20. The sacrificial layer of claim 11, wherein the layer includes
edges that correspond to the edges of the single substrate, and
wherein corners between the edges of the layer etch faster during
the epitaxial lift off process than the sides of the edges of the
layer.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. patent application
Ser. No. 15/876,001, filed on Jan. 19, 2018, which is a
continuation of U.S. patent application Ser. No. 13/210,138, filed
on Aug. 15, 2011. Each of these applications is incorporated herein
in its entirety by reference.
TECHNICAL FIELD
[0002] Embodiments of the invention generally relate to epitaxial
lift off (ELO) processes.
BACKGROUND
[0003] One phase in device fabrication involves handling and
packaging of thin films used in photovoltaic, semiconductor, or
other devices. Such thin film devices may be manufactured by
utilizing a variety of processes for depositing and removing
materials onto a wafer or other substrate. FIG. 1 shows a typical
wafer 101 that is used as a growth substrate. Wafer 101 has a flat
cut 103 which indicates its crystallographic orientation. This
wafer is typically diced into a rectangular or square configuration
105. An edge of the rectangle or square 105 aligns with that of a
flat cut 103. The edges of the rectangle or square 105 are either
parallel or perpendicular to flat cut 103. This is referred to as
an "on-axis" orientation.
[0004] One common technique for manufacturing thin film devices is
known as the epitaxial lift off (ELO) process. In an ELO process,
an epitaxial material or film, often containing multiple layers, is
formed on a sacrificial layer which is deposited on the growth
wafer. The sacrificial layer is etched away and the epitaxial film
is separate from the growth wafer. The isolated thin epitaxial film
may then be further processed and incorporated into a final
product, such as in photovoltaic, semiconductor, or other
devices.
[0005] The sacrificial layer is typically very thin and is usually
etched away via a wet chemical process. The speed of the overall
process may be limited by the lack of delivery or exposure of
reactant to the etch front, which leads to less removal of by
products from the etch front. The etching process is partially a
diffusion limited process, and if the films were maintained in
their deposited geometries, a very narrow and long opening would
form to severely limit the overall speed of the process. To lessen
the transport constraint of the diffusion processes, it may be
beneficial to open up the resulting gap created by the etched or
removed sacrificial layer and bending the epitaxial film away from
the growth wafer. A crevice is formed between the epitaxial film
and growth wafer, providing greater transport of species both
towards and away the etch front. Reactants move towards the etch
front while by-products generally move away from the etch
front.
[0006] Furthermore, the epitaxial films grown on nearly rectangular
or square growth wafers, or other wafers having sharp angles, are
especially susceptible to developing corner cracks during the ELO
processes. Basically, the diagonal corner regions experience
similar or a slightly slower etch rate compared to the straight
edges. This results in the corners of the sacrificial layer 201
becoming narrower, more pointed and pronounced, as depicted in FIG.
2. These narrower corners holding down the epitaxial film endure
correspondingly increased stresses when the epitaxial film is being
separated.
[0007] Therefore, there is a need for a method to remove epitaxial
films from the growth wafers without tearing the films during an
ELO process, as well as to maintain or increase the throughput of
the process.
SUMMARY
[0008] An off-axis epitaxial lift off process is disclosed. The
growth wafer has edges oriented in a direction other than that of
the natural cleavage plane. The corners of the growth wafer are
etched at a faster rate relative to the edges. This results in less
stress induced on the corners and thereby reduced corner cracking.
This increases the yield.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The accompanying drawings are incorporated in and form a
part of this specification. The drawings illustrate embodiments.
Together with the description, the drawings serve to explain the
principles of the embodiments.
[0010] FIG. 1 shows a prior art on-axis oriented wafer.
[0011] FIG. 2 shows a prior art on-axis orientation of a wafer as
it is being etched.
[0012] FIG. 3 shows an off-axis oriented wafer.
[0013] FIG. 4 shows an off-axis orientation of a wafer as it is
being etched.
DETAILED DESCRIPTION
[0014] Embodiments described herein provide processes for forming
and removing epitaxial films and materials from growth wafers by
epitaxial lift off (ELO) processes. In some embodiments, the growth
wafer has edge surfaces with an off-axis orientation which is
utilized during the ELO process. Off-axis pertains to the rotation
of an edge of the wafer from the nearest <110> orientation.
The off-axis orientation of the edge surface provides an additional
variable for controlling the etch rate during the ELO process--and
therefore the etch front may be modulated to prevent the formation
of high stress points between the epitaxial film stack and the
growth wafer which reduces or prevents stressing and cracking the
epitaxial film stack.
[0015] FIG. 3 shows an off-axis orientation of a wafer 301. The
circular wafer 301 has a flat cut 302 that indicates its
crystallographic orientation. Sometimes a notch will be used to
indicate the crystallographic orientation of the circular wafer
301. This circular wafer is diced or cut into a rectangle or square
303, upon which the ELO process is performed. Alternatively, a
rectangular or square crystal growth wafer 303 is diced out of the
crystal boule. In other words, the edges of the rectangular or
square growth wafer 303 are non-parallel and non-perpendicular to
the cleavage plane as identified by the primary flat cut 302 or
notch. The growth wafer has an edge that is oriented in a direction
other than the natural cleavage plane. In the currently preferred
embodiment, the rectangle or square growth wafer has edges at a 45
degree angle relative to the inherent crystallographic orientation
as indicated by the flat cut 302. By dicing the growth wafer at an
angle different from that of its natural, inherent crystallographic
orientation, the orientation of the growth wafer can be controlled
to take advantage of the different etch rates. In particular, the
area(s) more susceptible to stresses and cracking can be oriented
towards the faster etch rates. For rectangles and squares, the
corners can be oriented to point towards the faster etch
fronts.
[0016] FIG. 4 shows a growth wafer having corners that experience
relatively faster etch rates and edges that experience relatively
slower etch rates. The square growth wafer 401 is off-axis by
approximately 45 degrees. By virtue of this off-axis configuration,
the corners are etched at a faster rate relative to its edges. In
the currently preferred embodiment, the goal is to have corner etch
rate of at least 1.4 times that of the edge etch rate. Because the
corners are etched away faster, they become more rounded. The
rounded corners of the sacrificial layer 402 are far more desirable
than the narrower, more pointed corners of the prior art because
the rounded corners induce less stress during the lift-off process
and the epitaxial layer is less susceptible to cracking. This
improves the yield.
[0017] In embodiment herein, the method includes growing a
sacrificial layer over a growth wafer, forming an epitaxial film
stack over the sacrificial layer, and exposing the sacrificial
layer to a wet etch solution during the ELO process. The ELO
process includes etching the sacrificial layer, forming a crevice
between the growth wafer and the epitaxial film stack, and
separating the growth wafer from the epitaxial film stack.
[0018] The ELO etch process is a lateral etch process and the
geometry or shape of the etch front may be modulated as a function
of multiple variables including the crystalline lattice
orientation, etch chemistry (e.g. solution composition), etch
conditions (e.g., temperature and pressure), and curvature of the
crevice (e.g., dynamic clamping). The growth wafers described
herein have been specifically designed and created in order to have
control of the lattice orientation at specified locations on the
substrate. An off-axis orientation having a predetermined angle may
be used to change the etch rate at the corners and sides of the
sacrificial layer during the ELO process. For example, the off-axis
orientation of the growth substrate provides faster etching planes
at the corners of the sacrificial layer. Therefore, the corners may
be etched at a faster rate than the sides of the sacrificial layer
in order to provide that the etch front, coming from the corners
and sides having corner and side fronts, to converge near the
center of the substrate around the same time while forming a
singularity between the epitaxial film stack and the growth
wafer.
[0019] The growth wafers described herein which have edge surfaces
with an off-axis orientation rotated by a predetermined angle may
be manufactured or otherwise formed from a variety of different
growth wafers. A crystalline wafer may contain various elements,
including from Groups III, IV, and V, and initially may have a
variety of different crystalline orientations.
[0020] In one example, a rectangular growth wafer is cut off-axis
from a circular, crystalline, gallium arsenide wafer having a
facial orientation of <001>.
[0021] The predetermined angle for the rotated off-axis orientation
is measured from the <110> flat. The round wafer may be cut
with a saw, such as a dice saw, or other cutting or slicing device
used to cut crystalline wafers. The growth wafer is cut from the
round wafer at the predetermined angle relative to the edge flat,
such that the previously axis orientation is now rotated by the
predetermined angle. The predetermined angle is greater than
0.degree. and less than 90.degree..
[0022] In embodiments herein, the method includes exposing the
sacrificial layer to a wet etch solution during the ELO process.
The ELO process includes etching the sacrificial layer, forming a
crevice between the growth wafer and the epitaxial film stack, and
separating the growth wafer from the epitaxial film stack.
[0023] In another embodiment, the method further includes forming
an etch front by exposing the sacrificial layer to the wet etch
solution, wherein the etch front encompasses the sacrificial layer
at the interface of the wet etch solution and the sacrificial
layer. Prior to being exposed to the wet etch solution, the
sacrificial layer has side edges and corners which form a
rectangular geometry (e.g., rectangle or square). Once the
sacrificial layer is exposed to the wet etch solution, the etch
front advances towards the center of the growth wafer within the
crevice while etching the sacrificial layer during the ELO process.
The etch geometry of the sacrificial layer may be controlled to
transitions to have substantially octagonal geometry and then to
have substantially rounded geometry.
[0024] The etch front may have a rectangular geometry, conformal to
the sacrificial layer when initially exposed. However, as the
etching process progressed, the etch front generally forms an
octagonal geometry containing alternating sides of side edge fronts
and corner edge fronts. The growth wafer, as described herein
having an edge surface with an off-axis orientation rotated by an
angle greater than 0.degree. and up to 90.degree., is utilized to
etch the sacrificial layer at the corner edge fronts at a faster
etch rate than at the side edge fronts. Therefore, the side edge
fronts may be longer than the corner edge fronts during an initial
duration of the ELO process, but subsequently, the side edge fronts
and the corner edge fronts have the same length or substantially
the same length during a later duration of the ELO process. The
geometry of the etch front or the sacrificial layer are controlled
and modulated between transitions during the ELO process. The
geometry of the etch front or the sacrificial layer may transition
from a substantially rectangular geometry, to a substantially
octagonal geometry, and then to a substantially rounded geometry,
such as a rounded singularity. In some examples, the substantially
octagonal geometry of the etch front or the sacrificial layer
transitions from a non-equilateral octagonal geometry to an
equilateral or substantially equilateral octagonal geometry.
[0025] From the foregoing, it will be appreciated that specific
embodiments of the invention have been described herein for
purposes of illustration, but that various modifications may be
made without deviating from the scope of the invention.
Accordingly, the invention is not limited except as by the appended
claims.
* * * * *