U.S. patent application number 17/254623 was filed with the patent office on 2021-04-29 for systems and methods for growth of silicon carbide over a layer comprising graphene and/or hexagonal boron nitride and related articles.
This patent application is currently assigned to Massachusetts Institute of Technology. The applicant listed for this patent is The Government of the United States of America, as Represented by the Secretary of the Navy, Massachusetts Institute of Technology, ROHM Co., Ltd.. Invention is credited to David Kurt Gaskill, Jeehwan Kim, Wei Kong, Takuji Maekawa, Noriyuki Masago, Rachael L. Myers-Ward, Kuan Qiao.
Application Number | 20210125826 17/254623 |
Document ID | / |
Family ID | 1000005358146 |
Filed Date | 2021-04-29 |
![](/patent/app/20210125826/US20210125826A1-20210429\US20210125826A1-2021042)
United States Patent
Application |
20210125826 |
Kind Code |
A1 |
Myers-Ward; Rachael L. ; et
al. |
April 29, 2021 |
SYSTEMS AND METHODS FOR GROWTH OF SILICON CARBIDE OVER A LAYER
COMPRISING GRAPHENE AND/OR HEXAGONAL BORON NITRIDE AND RELATED
ARTICLES
Abstract
Systems and methods for growth of silicon carbide over a layer
comprising graphene and/or hexagonal boron nitride, and related
articles, are generally described. In some embodiments, a SiC film
is fabricated over a layer comprising graphene and/or hexagonal
boron nitride, which in turn is disposed over a substrate. The
layer and/or the substrate may be lattice-matched with the SiC film
to reduce defect density in the SiC film. The fabricated SiC film
may then be removed from the substrate via, for example, a stressor
attached to the SiC film. In certain cases, the layer serves as a
reusable platform for growing SiC films and also serves a release
layer that allows fast, precise, and repeatable release at the
layer surface.
Inventors: |
Myers-Ward; Rachael L.;
(Arlington, VA) ; Kim; Jeehwan; (Cambridge,
MA) ; Qiao; Kuan; (Cambridge, MA) ; Kong;
Wei; (Cambridge, MA) ; Gaskill; David Kurt;
(Alexandria, VA) ; Maekawa; Takuji; (Kyoto,
JP) ; Masago; Noriyuki; (Kyoto, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Massachusetts Institute of Technology
The Government of the United States of America, as Represented by
the Secretary of the Navy
ROHM Co., Ltd. |
Cambridge
Arlington
Ukyo-ku |
MA
VA |
US
US
JP |
|
|
Assignee: |
Massachusetts Institute of
Technology
Cambridge
MA
The Government of the United States of America, as Represented
by the Secretary of the Navy
Arlington
VA
ROHM Co., Ltd.
Ukyo-ku
|
Family ID: |
1000005358146 |
Appl. No.: |
17/254623 |
Filed: |
June 21, 2019 |
PCT Filed: |
June 21, 2019 |
PCT NO: |
PCT/US2019/038461 |
371 Date: |
December 21, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62688472 |
Jun 22, 2018 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 21/02458 20130101;
H01L 21/02598 20130101; H01L 21/02529 20130101; H01L 21/0262
20130101; H01L 21/02433 20130101; H01L 21/02389 20130101; H01L
21/02378 20130101; H01L 21/02444 20130101; H01L 21/02502
20130101 |
International
Class: |
H01L 21/02 20060101
H01L021/02 |
Claims
1. A method, comprising: forming a silicon carbide (SiC) film over
a layer comprising graphene and/or hexagonal boron nitride (hBN)
that is over a substrate; wherein at least a portion of the
formation of the SiC film occurs in the presence of a gaseous
material comprising an inert gas.
2. The method of claim 1, wherein the inert gas comprises Argon,
Helium, and/or nitrogen (N.sub.2).
3. The method of claim 1, further comprising separating the SiC
film and the substrate.
4. The method of claim 1, wherein the SiC film is crystalline.
5. The method of claim 1, wherein the SiC film is
single-crystalline.
6. The method of claim 1, further comprising forming the layer over
the substrate.
7. The method of claim 6, wherein forming the layer over the
substrate comprises growing the layer over the substrate.
8. The method of claim 6, wherein the substrate is a first
substrate, and forming the layer over the first substrate comprises
transferring the layer from a second substrate to the first
substrate.
9. The method of claim 1, wherein the layer is one of a plurality
of layers comprising graphene and/or hexagonal boron nitride (hBN),
and forming the SiC film comprises forming the SiC film over the
plurality of layers.
10. The method of claim 1, wherein the layer is the only layer
between the SiC film and the substrate.
11. The method of claim 1, wherein the layer is a
single-crystalline layer.
12. The method of claim 1, wherein the layer is a polycrystalline
layer.
13. The method of claim 1, wherein forming the SiC film over the
layer comprises using the substrate as a seed for the SiC film.
14. The method of claim 1, wherein, during the separating, the
layer is used as a release layer.
15. The method of claim 1, wherein forming the SiC film over the
layer comprises using the layer as a seed for the SiC film.
16. The method of claim 1, wherein forming the SiC film over the
layer comprises using a combination of the substrate and the layer
as a seed for the SiC film.
17. The method of claim 1, wherein the substrate is made, in whole
or in part, of SiC.
18. The method of claim 1, wherein the surface of the substrate
over which the layer is positioned during growth is an SiC
surface.
19. The method of claim 3, wherein separating the SiC film and the
substrate comprises exfoliating the SiC film.
20. The method of claim 3, wherein the SiC film is a first SiC
film, and further comprising forming a second SiC film over the
substrate after the first SiC film and the substrate have been
separated.
21. The method of claim 1, wherein the substrate is a semiconductor
substrate.
22. The method of claim 1, wherein the layer is directly on the
substrate.
23. The method of claim 1, wherein the substrate is made, in whole
or in part, of SiC having an offcut angle of between or equal to
0.degree. and 10.degree..
24. The method of claim 1, wherein the substrate is made, in whole
or in part, of 4.degree. off-axis SiC.
25. The method of claim 1, wherein the substrate is made, in whole
or in part, of 2.degree. off-axis SiC.
26. The method of claim 1, wherein the substrate is made, in whole
or in part, of on-axis SiC.
27. The method of claim 1, wherein the substrate is made, in whole
or in part, of (0001) 4H--SiC and/or (0001) 6H--SiC.
28. The method of claim 1, further comprising ramping, to a growth
temperature, an environment comprising a gaseous material
comprising an inert gas, wherein the environment is the environment
of the layer comprising the graphene and/or hBN over the substrate,
with a flow rate of the gaseous material of between or equal to 10
standard liters per minute (slm) and 80 slm.
29. The method of claim 1, wherein the at least one portion of the
formation of the silicon carbide film occurs at a growth
temperature, of an environment of the layer comprising the graphene
and/or hBN over the substrate, of between or equal to 1350.degree.
C. and 1800.degree. C.
30. The method of claim 1, wherein the gaseous material comprising
the inert gas is present during the at least one portion of the
formation of the silicon carbide film for between or equal to 3
minutes and 90 minutes.
31. The method of claim 1, wherein the substrate is made, in whole
or in part, of a III-Nitride.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C. .sctn.
119(e) to U.S. Provisional Patent Application Ser. No. 62/688,472,
filed Jun. 22, 2018, and entitled "REDUCTION OF BASAL PLANE
DISLOCATIONS IN EPITAXIAL SIC USING AN IN-SITU ETCH PROCESS," which
is incorporated herein by reference in its entirety for all
purposes.
FIELD
[0002] Systems and methods for growth of silicon carbide over a
layer comprising graphene and/or hexagonal boron nitride, and
related articles, are generally described.
BACKGROUND
[0003] In advanced electronic and photonic technologies, devices
are usually fabricated from functional semiconductors, such as
compound semiconductors including at least two chemical elements.
The lattice constants of these functional semiconductors typically
do not match the lattice constants of silicon substrates. As
understood in the art, lattice constant mismatch between a
substrate and an epitaxial layer (epilayer) on the substrate can
introduce strain into the epitaxial layer, thereby preventing
epitaxial growth of thicker layers without defects. Therefore,
non-silicon (non-Si) substrates are usually employed as seeds for
epitaxial growth of most functional semiconductors. However, non-Si
substrates with lattice constants matching those of functional
materials can be costly and therefore limit the development of
non-Si electronic/photonic devices.
[0004] One method to address the high cost of non-silicon
substrates is the "layer-transfer" technique, in which functional
device layers are grown on lattice-matched substrates and then
removed and transferred to other substrates. The remaining
lattice-matched substrates can then be reused to fabricate another
device layer, thereby reducing the cost. To significantly reduce
manufacturing costs, it can be desirable for a layer-transfer
method to have the following properties: 1) substrate reusability;
2) a minimal substrate refurbishment step after the layer release;
3) a fast release rate; and 4) precise control of release
thickness.
[0005] Conventional methods to remove and transfer a device layer
from a lattice-matched substrate include chemical lift-off (also
referred to as epitaxial lift-off or ELO), optical lift-off (also
referred to as laser lift-off or LLO, and mechanical lift-off (also
referred to as controlled spalling). Unfortunately, none of these
methods has the four desired properties at the same time.
[0006] Despite its continuous development over the last three
decades, chemical lift-off still has several disadvantages. For
example, the release rate is slow owing to slow penetration of
chemical etchant through the sacrificial layer (e.g., typically a
few days to release a single 8-inch wafer). Second, etching
residues tend to become surface contamination after release. Third,
chemical lift-off has limited reusability owing to the chemical
mechanical planarization (CMP) performed after release to recover
the roughened substrate surface into an epi-ready surface. Fourth,
it can be challenging to handle released epilayers in the chemical
solution.
[0007] The optical lift-off technique usually uses a high-power
laser to irradiate the back of the lattice-matched substrate (e.g.,
a transparent sapphire or SiC substrate) and selectively heat the
device-substrate interface, causing decomposition of the interface
and release of the device layer.
[0008] However, optical lift-off has its own limitations. First,
because the molten device film/substrate interface can make the
substrate rough, a reconditioning step is usually carried out
before reuse, thereby reducing the reusability to less than five
times. Second, local pressurization at the interface caused by
high-power thermal irradiation can induce cracks or dislocations.
Third, the laser scanning speed can be too slow to permit
high-throughput.
[0009] Controlled spalling can have a higher throughput than
optical lift-off. In this technique, high-stress films (also
referred to as "stressors") are deposited on the epitaxial film,
inducing fracture below the epilayers and resulting in the
separation of active materials from the substrate. When sufficient
tensile stress is applied to the interface, a Ku shear mode can
initiate a crack and a Ki opening mode can allow the propagation of
the crack parallel to the interface between the epilayer and the
substrate. By controlling the internal stress and thickness of the
stressor, strain energy sufficient to reach the critical Ki can be
provided, leading to fracture of the film/substrate interface.
Because the exfoliation occurs via crack propagation, the spalling
process can cause rapid release of films.
[0010] However, controlled spalling is not mature enough to be used
for commercial manufacturing for at least the following reasons.
First, because crack propagation generally occurs through cleavage
planes that are not always aligned normal to the surface, the
surface may need polishing for reuse. Second, a thick stressor is
usually used to provide enough energy to separate strong covalent
bonds, particularly when working with high Young's modulus
materials like compound semiconductors. Third, the internal stress
of the stressor may only be controlled in a narrow range, which
constrains the achievable thickness of the resulting spalled
film.
[0011] Accordingly, improved materials and methods are needed.
SUMMARY
[0012] Systems and methods for growth of silicon carbide over a
layer comprising graphene and/or hexagonal boron nitride, and
related articles, are generally described.
[0013] In some aspects, methods are provided. In some embodiments,
a method comprises forming a silicon carbide (SiC) film over a
layer comprising graphene and/or hexagonal boron nitride (hBN) that
is over a substrate; wherein at least a portion of the formation of
the SiC film occurs in the presence of a gaseous material
comprising an inert gas.
[0014] Other advantages and novel features of the present invention
will become apparent from the following detailed description of
various non-limiting embodiments of the invention when considered
in conjunction with the accompanying figures. In cases where the
present specification and a document incorporated by reference
include conflicting and/or inconsistent disclosure, the present
specification shall control.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Non-limiting embodiments of the present invention will be
described by way of example with reference to the accompanying
figures, which are schematic and are not intended to be drawn to
scale. In the figures, each identical or nearly identical component
illustrated is typically represented by a single numeral. For
purposes of clarity, not every component is labeled in every
figure, nor is every component of each embodiment of the invention
shown where illustration is not necessary to allow those of
ordinary skill in the art to understand the invention. In the
figures:
[0016] FIG. 1A is a schematic illustration of an exemplary article
1000 comprising a substrate 106 and a layer 104 comprising graphene
and/or hexagonal boron nitride (hBN), according to one set of
embodiments;
[0017] FIG. 1B is a schematic illustration of an exemplary system
2000 for growth of a silicon carbide (SiC) film 102 over substrate
106, according to one set of embodiments;
[0018] FIG. 1C is a schematic illustration of an exemplary system
3000 depicting separation of a silicon carbide film 102 from
substrate 106, according to one set of embodiments;
[0019] FIG. 1D is a schematic illustration of an exemplary system
4000 comprising a free-standing silicon carbide film 102 removed
from a substrate 106 (e.g., by separation as in FIG. 1C), according
to one set of embodiments;
[0020] FIG. 2A is a Nomarski micrograph of a SiC film grown on what
was originally (graphene layer)/SiC, with a ramping to growth
temperature conducted in hydrogen (H.sub.2) over an on-axis
substrate, according to one set of embodiments;
[0021] FIG. 2B is a Nomarski micrograph of a SiC film grown on what
was originally (graphene layer)/SiC, with a ramping to growth
temperature conducted in H.sub.2 over a 4.degree. off-axis
substrate, according to one set of embodiments;
[0022] FIG. 3A is a Nomarski micrograph of a SiC film grown by
remote epitaxy over an on-axis SiC substrate, with a graphene layer
in between the substrate and the growing film, with the SiC film
grown for 4 min, according to one set of embodiments;
[0023] FIG. 3B is a Nomarski micrograph of a SiC film grown by
remote epitaxy over an on-axis SiC substrate, with a graphene layer
in between the substrate and the growing film, with the SiC film
grown for 1 hr, according to one set of embodiments;
[0024] FIG. 4A is a Nomarski micrograph of a sample ramped in Argon
(Ar) and grown in Ar for 10 min and then 30 min in H.sub.2 at
1450.degree. C., according to one set of embodiments;
[0025] FIG. 4B is a Nomarski micrograph of a sample ramped in Ar
and grown in Ar for 1 hr at 1540.degree. C., according to one set
of embodiments;
[0026] FIG. 4C is a Nomarski micrograph of a sample ramped in Ar
and grown in Ar for 10 min at 1400.degree. C., according to one set
of embodiments;
[0027] FIG. 4D is a Nomarski micrograph of a sample ramped in Ar
and grown in Ar for 10 min at 1540.degree. C., according to one set
of embodiments;
[0028] FIG. 5A is a scanning electron microscopy (SEM) image of
surface morphology of a sample grown with growth conditions from
FIG. 4A, according to one set of embodiments;
[0029] FIG. 5B is an in-plane Electron Backscatter Diffraction
(EBSD) image of a sample grown with growth conditions from FIG. 4A,
according to one set of embodiments;
[0030] FIG. 5C is an out-of-plane EBSD image of a sample grown with
growth conditions from FIG. 4A, according to one set of
embodiments;
[0031] FIG. 5D is a SEM image of an exfoliated SiC film sample
grown with growth conditions from FIG. 4A, according to one set of
embodiments;
[0032] FIG. 6 is a high resolution transmission electron microscopy
(HRTEM) image (top) showing pseudo graphene (see, e.g., bottom
reference figure) after SiC film growth by remote epitaxy,
according to one set of embodiments;
[0033] FIG. 7A is a SEM image of a sample grown at 1620.degree. C.
for 20 min in 50 slm Ar over a 4.degree. off-axis substrate,
according to one set of embodiments;
[0034] FIG. 7B is an EBSD image of a sample grown at 1620.degree.
C. for 20 min in 50 slm Ar over a 4.degree. off-axis substrate,
according to one set of embodiments;
[0035] FIG. 7C is a SEM image of a sample grown at 1620.degree. C.
for 20 min in 50 slm Ar over a 2.degree. off-axis substrate,
according to one set of embodiments;
[0036] FIG. 7D is an EBSD image of a sample grown at 1620.degree.
C. for 20 min in 50 slm Ar over a 2.degree. off-axis substrate,
according to one set of embodiments;
[0037] FIG. 7E is a SEM image of a sample grown at 1620.degree. C.
for 20 min in 50 slm Ar over an on-axis substrate, according to one
set of embodiments;
[0038] FIG. 7F is an EBSD image of a sample grown at 1620.degree.
C. for 20 min in 50 slm Ar over an on-axis substrate, according to
one set of embodiments;
[0039] FIG. 8A is an X-ray Diffraction (XRD) rocking curve of the
(002) and (004) for SiC grown over a 4.degree. off-axis substrate,
according to one set of embodiments;
[0040] FIG. 8B is an XRD reciprocal space map (RSM) of the (004)
for SiC grown over a 4.degree. off-axis substrate, according to one
set of embodiments;
[0041] FIG. 8C is an XRD rocking curve of the (002) and (004) for
SiC grown over a 2.degree. off-axis substrate, according to one set
of embodiments;
[0042] FIG. 8D is an XRD RSM of the (004) for SiC grown over a
2.degree. off-axis substrate, according to one set of
embodiments;
[0043] FIG. 8E is an XRD rocking curve of the (002) and (004) for
SiC grown over an on-axis substrate, according to one set of
embodiments;
[0044] FIG. 8F is an XRD RSM of the (004) for SiC grown over an
on-axis substrate, according to one set of embodiments;
[0045] FIG. 9 is transmission electron microscopy (TEM) images
(left and right), and an SEM image (center) corresponding to FIG.
7A, of an SiC film grown by remote epitaxy on a 4.degree. off-axis
substrate in Ar at 1620.degree. C., according to one set of
embodiments;
[0046] FIG. 10A is a photograph of an exfoliated SiC film grown by
remote epitaxy at 1620.degree. C. in Ar over a 4.degree. off-axis
substrate, according to one set of embodiments;
[0047] FIG. 10B is an SEM image ("sub", top) of a substrate after
exfoliation and an SEM image ("tape", bottom) of an exfoliated SiC
film grown by remote epitaxy at 1620.degree. C. in Ar over a
4.degree. off-axis substrate, according to one set of
embodiments;
[0048] FIG. 10C is a photograph of an exfoliated SiC film grown by
remote epitaxy at 1620.degree. C. in Ar over a 2.degree. off-axis
substrate, according to one set of embodiments;
[0049] FIG. 10D is an SEM image ("sub", top) of a substrate after
exfoliation and an SEM image ("tape", bottom) of an exfoliated SiC
film grown by remote epitaxy at 1620.degree. C. in Ar over a
2.degree. off-axis substrate, according to one set of
embodiments;
[0050] FIG. 10E is a photograph of an exfoliated SiC film grown by
remote epitaxy at 1620.degree. C. in Ar over an on-axis substrate,
according to one set of embodiments;
[0051] FIG. 10F is an SEM image ("sub", top) of a substrate after
exfoliation and an SEM image ("tape", bottom) of an exfoliated SiC
film grown by remote epitaxy at 1620.degree. C. in Ar over an
on-axis substrate, according to one set of embodiments;
[0052] FIG. 11A-FIG. 11D illustrate a method 100 of fabricating a
semiconductor device using a graphene-based and/or hBN-based layer
transfer process, according to one set of embodiments; and
[0053] FIG. 12A-FIG. 12F illustrate a method 300 of graphene-based
and/or hBN-based layer fabrication and transfer using a stressor
layer and tape, according to one set of embodiments.
DETAILED DESCRIPTION
[0054] Systems and methods for growth of silicon carbide over a
layer comprising graphene and/or hexagonal boron nitride, and
related articles, are generally described.
[0055] In some aspects, methods are provided. In certain
embodiments, a method comprises forming a silicon carbide (SiC)
film over a layer comprising graphene and/or hexagonal boron
nitride (hBN) that is over a substrate, wherein at least a portion
of the formation of the SiC film occurs in the presence of a
gaseous material comprising an inert gas. In certain embodiments,
carrying out at least a portion of the formation of the silicon
carbide film in the presence of an inert gas advantageously
prevents or mitigates the etching of the layer (e.g., of the
graphene and/or of the hBN) during the at least one portion of the
formation of the silicon carbide film, such that the silicon
carbide film is formed over the layer rather than directly on the
substrate. In some such embodiments, the layer may advantageously
facilitate separation (e.g., mechanical separation) of the silicon
carbide film from the substrate after formation of the silicon
carbide film. Separation of the silicon carbide film from the
substrate may take place, in some embodiments, by first growing a
stressor and/or attaching tape over at least a portion of (e.g.,
all of) an exposed surface of the silicon carbide film, and then
mechanically separating the silicon carbide film from the substrate
by a force applied to the stressor and/or tape to pull at least a
portion of the stressor and/or tape away from the substrate. As
used herein, the term "tape" refers to a film comprising an
adhesive that adheres to silicon carbide and/or adheres to a
stressor material (e.g., nickel). In certain embodiments, the
separated silicon carbide film may then advantageously be
transferred to a second substrate (e.g., comprising a
semiconductor, e.g., silicon), e.g., for fabrication of a device
comprising the silicon carbide film.
[0056] As described above, it may be desirable for methods (e.g.,
layer-transfer processes) to have at least substrate reusability,
minimal need for post-release treatment, a fast release rate,
and/or precise control of release interfaces. Conventional
layer-transfer processes may exhibit some of the desired
properties. For example, layer release may be much faster for
mechanical lift-off than for chemical or optical lift-off, whereas
the release location may be better controlled in chemical and
optical lift-off. However, conventional layer-transfer methods may
suffer from rough surface formation after layer release, thereby
limiting substrate reusability. In fact, the process cost to
refurbish a substrate surface in conventional layer-transfer
methods typically exceeds the substrate cost, so practical
applications in manufacturing can be challenging.
[0057] To address the shortcomings in conventional layer-transfer
methods, certain systems and methods described herein employ a
layer-transfer approach towards fabricating devices comprising a
SiC film. In some embodiments, a method comprises forming a SiC
film over a layer comprising graphene and/or hBN, which layer in
turn is disposed over a substrate that, in some embodiments, is
lattice-matched to the SiC film. In some embodiments, the method
comprises depositing the layer comprising graphene and/or hBN
directly on the substrate (e.g., a lattice-matched substrate). In
some embodiments, the method comprises transferring the layer
comprising graphene and/or hBN to the substrate (e.g.,
lattice-matched substrate) from another substrate. In some
embodiments, the method comprises removing the formed SiC film from
the substrate (e.g., lattice-matched substrate) via, for example, a
stressor attached to the SiC film.
[0058] In certain embodiments, in a non-limiting layer-transfer
method, a layer comprising graphene and/or hBN serves as a reusable
platform for growing SiC films and also serves as a release layer
that allows fast, precise, and repeatable release at the graphene
surface. Compared to conventional methods, a layer-transfer method
may have one or more of several advantages. First, because graphene
and/or hBN are crystalline films, they may be generally suitable as
a base for growing epitaxial over-layers. Second, weak interaction
of graphene and/or hBN with other materials may substantially relax
the lattice mismatching rule for epitaxial growth, facilitating the
growth of SiC films with low defect densities. Third, an SiC film
grown on a layer comprising graphene and/or hBN may be easily and
precisely released from the substrate over which the layer is
disposed, owing at least to weak van der Waals interactions of
graphene and/or hBN, which may facilitate rapid mechanical release
of SiC films grown over the layer without post-release
reconditioning of the released surface. Fourth, robustness of
graphene and/or hBN may facilitate reusability of a layer for
multiple growth/release cycles.
[0059] Implementation of methods described herein may have a
significant impact on both the scientific community and industry,
at least because layer-transfer methods using layers comprising
graphene and/or hBN have the potential to fabricate devices without
the expensive millimeter-thick, single-crystalline wafers used in
current semiconductor processing. Moreover, in certain embodiments,
SiC films can be transferred from a layer comprising graphene
and/or hBN, for additional flexible functions.
[0060] In some embodiments, a method comprises forming a silicon
carbide (SiC) film over a layer comprising graphene and/or
hexagonal born nitride (hBN) that is over a substrate. Embodiments
of silicon carbide films, embodiments of layers, and embodiments of
substrates are described in further detail elsewhere herein.
[0061] Terms that describe the relative spatial positions of
components (e.g., "above," "below," "over," "under," etc.) are
generally used herein for ease of description and understanding of
the relative arrangement of components, for example, as shown in
the accompanying figures. As would be understood by a person of
ordinary skill in the art, however, claims reciting such terms are
intended to encompass varying orientations of the articles
comprising those components, as long as the components are
positioned relative to each other in a manner consistent with the
recitations in the claims. For example, features described as being
"below" or "under" other features would, after the article is
turned upside down, be oriented "above" or "on top of" the other
features. As another example, features described as being "below"
or "under" other features would, after the article is rotated 90
degrees counter-clockwise, be oriented "to the right of" the other
features.
[0062] When a structure (e.g., layer and/or device) is referred to
as being "on," "over," or "overlying" another structure (e.g.,
layer or substrate), it can be directly on the structure, or an
intervening structure (e.g., a solid material, air gap, etc.) also
may be present. A structure that is "directly on" or "directly
over" or "in direct contact with" another structure, or placed
"directly onto" another structure, means that no intervening
structure is present. It should also be understood that when a
structure is referred to as being "on" or "over" another structure,
it may cover the entire structure, or a portion of the
structure.
[0063] In some embodiments, at least a portion of the formation of
the SiC film occurs in the presence of a gaseous material
comprising an inert gas. In some embodiments, the inert gas
comprises (e.g., in an amount of at least 10 at %, 25 at %, 50 at
%, 75 at %, 90 at %, 99 at %, or more) Argon (Ar), Helium (He),
and/or nitrogen (N.sub.2). As used herein, the term "inert gas"
refers to a gas or vapor comprising atoms or molecules that do not
chemically react with and/or etch the graphene and/or hexagonal
boron nitride. In certain embodiments, this utilization of inert
gas during at least a portion of the formation of the silicon
carbide film decreases or prevents etching of a layer over the
substrate, such that the at least one portion of the silicon
carbide film is formed over the layer. In some embodiments, the
inert gas consists essentially of Argon, Helium, and/or nitrogen
(N.sub.2). In certain embodiments, the inert gas comprises (e.g.,
in an amount of at least 10 at %, 25 at %, 50 at %, 75 at %, 90 at
%, or 99 at %) or consists essentially of Argon (Ar).
[0064] In certain embodiments, at least a portion of the duration
of the formation of the silicon carbide film occurs in the presence
of a gaseous material comprising an inert gas. In some embodiments,
the gaseous material comprising the inert gas is present for at
least 3 minutes, at least 5 minutes, or at least 10 minutes of the
formation of the silicon carbide film. In some embodiments, the
gaseous material comprising the inert gas is present for at most 90
minutes, at most 60 minutes, at most 40 minutes, or at most 20
minutes. Combinations of the above-referenced ranges are also
possible (e.g., between or equal to 3 minutes and 90 minutes,
between or equal to 5 minutes and 60 minutes, between or equal to
10 minutes and 20 minutes). Other ranges are also possible. In
certain embodiments, the gaseous material comprising the inert gas
is present for 3 minutes. In certain embodiments, the gaseous
material comprising the inert gas is present for 10 minutes. In
certain embodiments, the gaseous material comprising the inert gas
is present for 20 minutes. In certain embodiments, the gaseous
material comprising the inert gas is present for 60 minutes.
[0065] As used herein, the formation of the silicon carbide film
involves the introduction of or presence of precursor gases (e.g.,
silane and propane) to silicon carbide in an environment of a layer
comprising graphene and/or hBN over a substrate.
[0066] Methods described herein may involve forming a silicon
carbide film over a structure, comprising a layer over a substrate,
having a certain architecture. For example, in some embodiments,
the layer comprising the graphene and/or hexagonal boron nitride is
one of a plurality of layers, and forming the SiC film comprises
forming the SiC film over (e.g., directly on) the plurality of
layers comprising graphene and/or hexagonal boron nitride. In some
embodiments, the layer comprising the graphene and/or hexagonal
boron nitride is the only layer between the SiC film and the
substrate. In some embodiments, a single layer of graphene is used.
In certain embodiments, a single layer of hexagonal boron nitride
is used.
[0067] In certain embodiments, a method comprises growing a silicon
carbide film over a structure, comprising a layer over a substrate,
using a material of the structure as a seed for growth of the
silicon carbide film. For example, in some embodiments, forming the
SiC film on the layer comprises using the substrate as a seed for
the SiC film. In some embodiments, forming the SiC film on the
layer comprises using the layer comprising the graphene and/or
hexagonal boron nitride as a seed for the SiC film. In some
embodiments, forming the SiC film on the layer comprises using a
combination of the substrate and the layer comprising the graphene
and/or hexagonal boron nitride as a seed for the SiC film.
[0068] In some embodiments, the substrate is made, in whole or in
part, of a compound semiconductor. For example, in certain
embodiments, the substrate is made, in whole or in part, of a
III-Nitride, which comprises a group III element in the periodic
table (e.g., aluminum, gallium) and nitrogen. Non-limiting examples
of substrate materials include gallium nitride (GaN), aluminum
nitride (AlN), and aluminum gallium nitride (Al.sub.xGa.sub.1-xN).
In certain embodiments, the substrate comprises silicon carbide.
For example, in some embodiments, a surface of the substrate
comprises silicon carbide. In certain embodiments, the surface of
the substrate over which the layer is positioned during growth is
an SiC surface.
[0069] In some embodiments, a method comprises forming a layer
comprising graphene and/or hexagonal boron nitride on a substrate.
In some embodiments, forming the layer on the substrate comprises
growing the layer on the substrate. In some embodiments, the
substrate is a first substrate, and forming the layer on the first
substrate comprises transferring the layer from a second substrate
to the first substrate. In some embodiments, the layer is directly
on the substrate.
[0070] In certain embodiments, a method comprises forming a silicon
carbide film on a layer comprising graphene and/or hexagonal boron
nitride that is on a substrate after forming the layer on the
substrate.
[0071] In some embodiments, the method comprises ramping, to a
growth temperature, an environment comprising a gaseous material
comprising an inert gas, wherein the environment is the environment
of a layer comprising graphene and/or hBN over a substrate. In
certain embodiments, the ramping occurs before formation of the
silicon carbide film. In some embodiments, the method comprises
ramping, to a growth temperature, an environment comprising a
gaseous material comprising an inert gas, wherein the environment
is the environment of a layer comprising graphene and/or hBN over a
substrate, with a flow of the gaseous material at a high enough
flow rate to decrease, suppress, or eliminate growth of graphene or
additional graphene during the ramp. In some embodiments, the flow
rate of the gaseous material comprising the inert gas during the
ramp is greater than or equal to 10 standard liters per minute
(slm), greater than or equal to 20 slm, greater than or equal to 30
slm, or greater than or equal to 40 slm. In some embodiments, the
flow rate of the gaseous material comprising the inert gas during
the ramp is less than or equal to 80 slm, less than or equal to 70
slm, less than or equal to 60 slm, or less than or equal to 50 slm.
Combinations of the above-referenced ranges are also possible
(e.g., between or equal to 10 slm and 80 slm, between or equal to
20 slm and 70 slm, between or equal to 30 slm and 60 slm). Other
ranges are also possible. For example, in certain embodiments, the
flow rate of the gaseous material comprising the inert gas during
the ramp is 50 slm. In certain embodiments, the flow rate of the
gaseous material comprising the inert gas during the ramp is 30
slm.
[0072] In some embodiments, at least a portion of the formation of
the silicon carbide film occurs at a particular growth temperature
of the environment of a layer comprising graphene and/or hBN over a
substrate. In some embodiments, at least a portion of the formation
of the silicon carbide film occurs at a temperature of the
environment of at least 1350.degree. C., at least 1400.degree. C.,
or at least 1500.degree. C. In some embodiments, at least a portion
of the formation of the silicon carbide film occurs at a
temperature of the environment of at most 1800.degree. C., at most
1620.degree. C., or at most 1540.degree. C. Combinations of the
above-referenced ranges are also possible (e.g., between or equal
to 1350.degree. C. and 1800.degree. C., between or equal to
1350.degree. C. and 1500.degree. C., between or equal to
1500.degree. C. and 1800.degree. C., between or equal to
1400.degree. C. and 1540.degree. C.). Other ranges are also
possible. In certain embodiments, at least a portion of the
formation of the silicon carbide film occurs at a temperature of
the environment of 1620.degree. C. In certain embodiments, at least
a portion of the formation of the silicon carbide film occurs at a
temperature of the environment of 1400.degree. C. In certain
embodiments, at least a portion of the formation of the silicon
carbide film occurs at a temperature of the environment of
1540.degree. C.
[0073] In some embodiments, a method comprises separating a SiC
film from a substrate. In certain embodiments, during the
separating, the layer comprising the graphene and/or hexagonal
boron nitride is used as a release layer. In some embodiments,
separating the SiC film and the substrate comprises exfoliating the
SiC film. In some embodiments, the SiC film is a first SiC film,
and the method further comprises forming a second SiC film on the
substrate after the first SiC film and the substrate have been
separated.
[0074] In certain embodiments, a method comprises separating a
silicon carbide film from a substrate after forming a silicon
carbide film on a layer comprising graphene and/or hexagonal boron
nitride that is on a substrate. In certain embodiments, the method
comprises forming the silicon carbide film on the layer after
forming the layer on the substrate.
[0075] SiC films herein generally comprise at least some
crystallinity. For example, in some embodiments, the silicon
carbide film is crystalline. In certain embodiments, the SiC film
is single-crystalline. In some embodiments, the SiC film comprises
more than one polytype. For example, in some embodiments, the SiC
film comprises two polytypes. In certain embodiments, the SiC film
is polycrystalline.
[0076] In some embodiments, a film (e.g., SiC film) is a layer of
material ranging from 0.1 nm to 1000 .mu.m in thickness.
[0077] The layers between the substrate and the SiC described
herein generally comprise graphene and/or hBN. For example, in some
embodiments, the layer consists essentially of graphene and/or hBN.
In certain embodiments, the layer is a graphene layer. In certain
embodiments, the layer is a hBN layer.
[0078] The layers between the substrate and the SiC described
herein generally comprise at least some crystallinity. For example,
in some embodiments, the layer is a polycrystalline layer. In some
embodiments, the layer is a single-crystalline layer.
[0079] In certain embodiments, the substrate comprises a
semiconductor. For example, in some embodiments, the substrate is a
semiconductor substrate. In some embodiments, the substrate is
made, in whole or in part, of SiC.
[0080] In some embodiments, the substrate is made, in whole or in
part, of SiC having a certain offcut angle. In some embodiments,
the substrate is made, in whole or in part, of SiC having an offcut
angle of greater than or equal to 0.degree., greater than or equal
to 2.degree., or greater than or equal to 4.degree.. In some
embodiments, the substrate is made, in whole or in part, of SiC
having an offcut angle of less than or equal to 10.degree., less
than or equal to 8.degree., or less than or equal to 6.degree..
Combinations of the above-referenced ranges are also possible
(e.g., between or equal to 0.degree. and 10.degree., between or
equal to 0.degree. and 8.degree., between or equal to 0.degree. and
4.degree.). For example, in some embodiments, the substrate is
made, in whole or in part, of 4.degree. off-axis SiC. As used
herein, 4.degree. off-axis silicon carbide corresponds to a
4.degree. offcut angle. In some embodiments, the substrate is made,
in whole or in part, of 2.degree. off-axis SiC. As used herein,
2.degree. off-axis silicon carbide corresponds to a 2.degree.
offcut angle. In some embodiments, the substrate is made, in whole
or in part, of on-axis SiC. As used herein, on-axis silicon carbide
corresponds to a 0.degree. offcut angle.
[0081] Substrates herein generally comprise at least some
crystallinity. For example, in some embodiments, the substrate is
crystalline. In certain embodiments, the substrate is
single-crystalline.
[0082] Turning now to the figures, several non-limiting embodiments
are described in further detail. It should be understood that the
current disclosure is not limited to only those specific
embodiments described herein. Instead, the various disclosed
components, features, and methods may be arranged in any suitable
combination as the disclosure is not so limited.
[0083] FIG. 1A is a schematic illustration of an exemplary article
1000 comprising a substrate 106 and a layer 104 comprising graphene
and/or hexagonal boron nitride, according to one set of
embodiments. In certain embodiments, a method comprises forming
layer 104 on substrate 106. In other embodiments, a method
comprises transferring layer 104 to substrate 106.
[0084] FIG. 1B is a schematic illustration of an exemplary system
2000 for growth of a silicon carbide (SiC) film 102 over substrate
106, according to one set of embodiments.
[0085] In certain embodiments, a method comprises forming silicon
carbide film 102 over layer 104 comprising graphene and/or
hexagonal boron nitride that is over substrate 106, wherein at
least a portion of the formation of silicon carbide film 102 occurs
in the presence of a gaseous material 108 comprising an inert gas
comprising species 112, wherein species 112 is an atom or molecule
of an inert gas (e.g., helium, argon, nitrogen (N.sub.2)). In
certain embodiments, during the at least one portion of the
formation of silicon carbide film 102, gaseous material 108 further
comprises gaseous precursors to silicon carbide. In certain
embodiments, substrate 106 comprises silicon carbide.
[0086] FIG. 1C is a schematic illustration of an exemplary system
3000 depicting separation of silicon carbide film 102 from
substrate 106, according to one set of embodiments. In certain
embodiments, silicon carbide film 102 was formed by a method
described with respect to FIG. 1B. In certain embodiments, a method
comprises mechanically separating silicon carbide film 102 from
substrate 106. In some embodiments, mechanical separation of
silicon carbide film 102 from substrate 106 is accomplished by
first growing a stressor (not shown) and/or attaching tape (not
shown) to at least a portion of (e.g., all of) an exposed surface
of silicon carbide film 102 and then applying a force, directed
away from substrate 106, to at least a portion of the stressor
and/or tape.
[0087] FIG. 1D is a schematic illustration of an exemplary system
4000 comprising a free-standing silicon carbide film 102 removed
from substrate 106 (e.g., by separation as in FIG. 1C), according
to one set of embodiments. Free-standing silicon carbide film 102,
in some embodiments, may be disposed under a stressor and/or tape.
In certain embodiments, free-standing silicon carbide film 102 may
be transferred to another substrate, e.g., towards fabrication of a
device comprising silicon carbide film 102.
[0088] FIG. 11A-FIG. 11D illustrate a method 100 of fabricating a
SiC film using graphene and/or hBN as a platform, according to some
embodiments. As shown in FIG. 11A, a layer 120 comprising graphene
and/or hBN may be fabricated on a first substrate 110, such as a Si
substrate, SiC substrate, or copper foil. In certain embodiments,
layer 120 consists essentially of graphene and/or hBN. Fabricated
layer 120 may then be removed from first substrate 110 as shown in
FIG. 11B. Removed layer 120 may then be disposed on a second
substrate 130, such as a SiC substrate, as shown in FIG. 11C. FIG.
11D shows that a SiC film 140 (e.g., a single crystalline SiC film,
advantageous for high electrical and optical device performance)
may then be fabricated on layer 120. An SiC film (e.g., 140) may
also be referred to as a device layer or a functional layer in this
application.
[0089] Layer 120 may be fabricated on first substrate 110 via
various methods. In one example, layer 120 may include an epitaxial
graphene with a single-crystalline orientation and/or an epitaxial
hBN with a single-crystalline orientation and substrate 110 may
include (0001) 4H--SiC (e.g., with a silicon surface) and/or (0001)
6H--SiC (e.g., with a silicon surface). The fabrication of layer
120 may include multistep annealing steps. A first annealing step
may be performed in H.sub.2 gas for surface etching and
vicinalization, and a second annealing step may be performed in Ar
for graphitization at high temperature (e.g., about 1,575.degree.
C.).
[0090] In another example, layer 120 may be grown on first
substrate 110 via a chemical vapor deposition (CVD) process.
Substrate 110 may include a nickel substrate or a copper substrate.
Alternatively, substrate 110 may include an insulating substrate of
SiO.sub.2, HfO.sub.2, Al.sub.2O.sub.3, Si.sub.3N.sub.4, and/or
practically any other high temperature compatible planar material
by CVD.
[0091] In yet another example, first substrate 110 may be any
substrate that can hold layer 120 and the fabrication may include a
mechanical exfoliation process. In this example, first substrate
110 may function as a temporary holder for layer 120.
[0092] Various methods may also be used to transfer layer 120 from
first substrate 110 to second substrate 130. In one example, a
carrier film may be attached to layer 120. The carrier film may
include a thick film of Poly(methyl methacrylate) (PMMA) or a
thermal release tape and the attachment may be achieved via a
spin-coating process. In some embodiments, after the combination of
the carrier film and layer 120 disposed on second substrate 130,
the carrier film may be dissolved (e.g., in acetone) for further
fabrication of SiC film 140 on layer 120.
[0093] In another example, a stamp layer including an elastomeric
material such as polydimethylsiloxane (PDMS) may be attached to
layer 120 and first substrate 110 may be etched away, leaving the
combination of the stamp layer and layer 120. In some embodiments,
after the stamp layer and layer 120 are placed on second substrate
130, the stamp layer may be removed by mechanical detachment,
producing a clean surface of layer 120 for further processing.
[0094] In yet another example, a self-release transfer method may
be used to transfer layer 120 to second substrate 130. In this
method, a self-release layer may first be spun-cast over layer 120.
An elastomeric stamp may then be placed in conformal contact with
the self-release layer. First substrate 110 may be etched away to
leave the combination of the stamp layer, the self-release layer,
and layer 120. In some embodiments, after this combination is
placed on second substrate 130, the stamp layer may be removed
mechanically and the self-release layer may be dissolved under mild
conditions in a suitable solvent. The release layer may include
polystyrene (PS), poly(isobutylene) (PIB), and/or Teflon AF
(poly[4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole-co-tetr-
afluoroethylene]).
[0095] In one example, the lattice of second substrate 130 is
matched to SiC film 140, in which case second substrate 130
functions as the seed for the growth of SiC film 140 if layer 120
is thin enough (e.g., if layer 120 is one layer thick). Sandwiching
layer 120 between second substrate 130 and SiC film 140 may
facilitate quick and damage-free release and transfer of SiC film
140.
[0096] In another example, layer 120 may be thick enough (e.g.,
several layers thick) to function as a seed to grow SiC film 140,
in which case SiC film 140 may be lattice-matched to layer 120.
This example may also facilitate repeated use of second substrate
130. In yet another example, second substrate 130 together with
layer 120 may function as the seed to grow SiC film 140.
[0097] Using layer 120 as the seed to fabricate SiC film 140 may
also increase the tolerance over mismatch of lattice constant
between the SiC film layer 120.
[0098] Without wishing to be bound by any particular theory or mode
of operation, surfaces of graphene and/or hBN typically have no
dangling bonds and interact with material above them via weak van
der Waals like forces. Due to the weak interaction, a SiC film may
grow from the beginning with its own lattice constant forming an
interface with a small amount of defects. This kind of growth may
be referred to as Van Der Waals Epitaxy (VDWE). The lattice
matching condition may be drastically relaxed for VDWE, allowing a
large variety of different heterostructures even for highly lattice
mismatched systems.
[0099] In practice, the lattice mismatch may be about 0% to about
70% (e.g., about 0%, about 10%, about 20%, about 30%, about 40%,
about 50%, about 60%, and about 70%, including any values and sub
ranges in between).
[0100] In one example, SiC film 140 includes a 2D material system.
In another example, SiC film 140 includes a 3D material system. The
flexibility to fabricate both 2D and 3D material systems allows
fabrication of a wide range of optical, optoelectronic, and
photonic devices known in the art.
[0101] The fabrication of SiC film 140 may be carried out using
semiconductor fabrication technique known in the art. For example,
low-pressure Chemical Vapor Deposition (CVD) may be used to grow
SiC film 140 on layer 120, which in turn is disposed on second
substrate 130 (e.g., a SiC substrate). In this example, layer 120
and second substrate 130 may be baked (e.g., under H.sub.2 for
>15 min at >1,100.degree. C.) to clean the surface. Then the
deposition of SiC film 140 may be performed.
[0102] FIG. 12A-FIG. 12F illustrate a method 300 of layer transfer.
FIG. 12A shows that a layer 320 comprising graphene and/or hBN is
formed or disposed on a donor wafer 310, which may be a
single-crystalline wafer. For example, layer 320 may include
epitaxial graphene grown on donor wafer 310. Alternatively, layer
320 may be exfoliated and transferred to donor wafer 310 from
another wafer (not shown). In yet another example, any of the
transfer techniques described above with reference to FIG. 11A-FIG.
11D may be used here to prepare layer 320 disposed on donor wafer
310.
[0103] FIG. 12B shows that a SiC film 330 may be epitaxially grown
on layer 320. SiC film 330 may include an electronic layer, a
photonic layer, or any other functional device layer. Methods to
fabricate SiC film 330 may include any methods and techniques
described above with respect to FIG. 11A-FIG. 11D.
[0104] FIG. 12C shows that a stressor 340 may be disposed on SiC
film 330. For example, stressor 340 may include a high-stress metal
film such as a Ni film. In one example, the Ni stressor may be
deposited in an evaporator at a vacuum level of 1.times.10.sup.-5
Torr.
[0105] FIG. 12D shows that a tape layer 350 may be disposed on
stressor 340 for handling stressor 340. Using tape 350 and stressor
340 may mechanically exfoliate SiC film 330 from layer 320 at a
fast release rate by applying high strain energy to the interface
between SiC film 330 and layer 320. The release rate may be fast at
least due to the weak van der Waals bonding between graphene and
other materials such as SiC film 330.
[0106] In FIG. 12E, released SiC film 330, together with stressor
340 and tape layer 350 may be disposed on a host wafer 360. In FIG.
12F, tape 340 and stressor 340 may be removed, leaving SiC film 330
for further processing such as forming more sophisticated devices
or depositing additional materials on SiC film 330. In one example,
tape layer 350 and stressor 340 may be etched away by a
FeCl.sub.3-based solution.
[0107] In the method 300, after the release of SiC film 330 shown
in FIG. 12D, remaining donor wafer 310 and layer 320 may be reused
for a next cycle of SiC film fabrication. Alternatively, layer 320
may also be released. In this case, a new layer may be disposed on
donor wafer 310 before next cycle of SiC film fabrication. In
either case, layer 320 may protect donor wafer 310 from damage,
thereby allowing multiple uses and reducing cost.
[0108] In contrast, conventional processes usually include
chemical-mechanical planarization (CMP) after release to
recondition the wafer surface. CMP may consume relatively thick
materials, and repeated CMPs may increase the chance of breaking a
wafer. Graphene-based and/or hBN-based layer transfer may increase
reusability because layer transfer may involve an atomically smooth
release surface. In layer transfer, layer release may occur
precisely at the interface between SiC film 330 and layer 320 at
least because graphene's and/or hBN's weak van der Waals force
would not involve strong bonding to adjacent materials. This may
facilitate layer 320 to be reused for multiple growth/exfoliation
cycles without the need for a polishing step and without damaging
the graphene, due to its mechanical robustness. In addition, layer
transfer may ensure a fast release rate. Because SiC film 330 may
be mechanically released from a weakly bound layer 320 surface, the
layer release rate in layer transfer may be high.
[0109] Furthermore, in some embodiments, by having a highly
strained freestanding SiC film 330 after release as shown in FIG.
12D, devices comprising SiC film 330 may have higher electron or
hole mobility. An optoelectronic device comprising SiC film 330 may
also have an enhanced optical response.
[0110] For mechanical release of SiC film 330 from layer 320, it
may be desirable for the material of stressor 340 to provide enough
strain energy to the SiC film 330/layer 320 interfaces to promote
damage-free exfoliation/transfer. One concern for the mechanical
release process may be the bending of SiC film 330 during
exfoliation and self-exfoliation during deposition of stressor 340.
If the radius of curvature is reduced during exfoliation, strain
energy may increase in SiC film 330. In some embodiments, when the
strain energy reaches a critical point, cracks may form. Also, if
strain energy in the stressor exceeds the SiC film 330/layer 320
interface energy, SiC film 330 may be delaminated during stressor
deposition. To address this concern, the transfer of SiC film 330
on layers 320 may be performed using feedback loop control.
[0111] International Publication Number WO 2017/044577, published
Mar. 16, 2017, filed Sep. 8, 2016, and entitled "SYSTEMS AND
METHODS FOR GRAPHENE BASED LAYER TRANSFER," is incorporated herein
by reference in its entirety.
[0112] U.S. Provisional Patent Application Ser. No. 62/688,472,
filed Jun. 22, 2018, and entitled "REDUCTION OF BASAL PLANE
DISLOCATIONS IN EPITAXIAL SIC USING AN IN-SITU ETCH PROCESS," is
incorporated herein by reference in its entirety.
[0113] The following examples are intended to illustrate certain
embodiments described herein, including certain aspects of the
present invention, but do not exemplify the full scope of the
invention.
Example 1
[0114] This example generally describes the growth of SiC films,
e.g., by remote epitaxy.
[0115] A purpose of growing SiC films by remote epitaxy was to
transfer epitaxially grown SiC films from a (graphene layer)/SiC
substrate to a desired substrate. Growing SiC films by remote
epitaxy may facilitate making electronic heterostructures which
would otherwise be impractical to grow. Growing SiC films by remote
epitaxy may also facilitate creating devices at reduced costs by
transferring to less expensive substrates or transferring to a
material that may provide properties advantageous for a specific
device. For example, a SiC film grown by remote epitaxy may be used
to create a photonic crystal cavity for quantum sciences, e.g., by
transferring a SiC film grown by remote epitaxy to a SiO.sub.2/Si
substrate, resulting in a more easily processed cavity, at least
due to working with Si instead of SiC as the substrate. Growing SiC
films by remote epitaxy may also facilitate processing of the
material compared to SiC processing. One may also use a SiC film
grown by remote epitaxy to make power devices at lower cost to the
consumer at least because the active region of the device may be
still be SiC while the substrate may be Si for example, potentially
making a significant impact on cost reduction due at least to
reduced cost of the substrate. In addition to the ability to remove
and/or transfer SiC films grown by remote epitaxy, a SiC film that
is grown by remote epitaxy may also have a lower dislocation
density and be of higher quality than the substrate, at least
because the dislocations may not propagate as readily through a
graphene layer (or hexagonal born nitride (hBN) layer) into the SiC
film grown by remote epitaxy. Additionally, it may be difficult to
grow a SiC film over an on-axis substrate. Using a graphene layer
between a SiC substrate and a SiC film grown by remote epitaxy
improved the surface morphology, relative to a silicon carbide film
grown directly on an on-axis silicon carbide substrate, and
resulted in a single crystal SiC film.
[0116] Growth of a SiC film, not by remote epitaxy, took place by
first flowing a carrier gas, typically H.sub.2, and ramping the
temperature to a growth temperature. A typical process began by
flowing 80 slm H.sub.2 over a substrate with a pressure at 100
mbar. Measurement in standard liters per minute (slm) assumed
standard temperature and pressure (60.degree. F. (about
15.56.degree. C.) and 1 atm). Then, the temperature (T) was ramped
to the desired growth T (e.g., 1620.degree. C.). Once the growth
temperature was reached, the temperature, pressure, and flow
rate(s) were stabilized for 5 min to allow any polishing damage or
contaminants to be etched away from the substrate, which damage or
contaminants may have been on the surface of the substrate, to
prepare the surface for growth. This is a reason H.sub.2 was
typically the carrier gas. After this time, precursors were
introduced where silane was the silicon (Si) source and propane was
the carbon (C) source. The silane was introduced anywhere from 300
sccm to 2000 sccm with a C/Si ratio of 1.5. Measurement in standard
cubic centimeters per minute (sccm) assumed standard temperature
and pressure (60.degree. F. (about 15.56.degree. C.) and 1 atm).
Here, H.sub.2 was the carrier gas, e.g., to facilitate the propane
to crack into the necessary elements for growth. The flows, T, and
pressure (P) were maintained during growth for a desired amount of
time. After growth, the precursors were turned off and the sample
was cooled in H.sub.2. Again, one reason to have H.sub.2 flowing
during the ramp was to prepare the surface of the substrate for
growth.
[0117] To attempt to grow SiC by remote epitaxy, a SiC substrate
first went through Si sublimation to form a graphene layer. A SiC
film was grown on pseudo graphene that had been exfoliated with Ni
metal to remove the top layer of graphene and leave the 6 3.times.6
3 reconstructed SiC. However, as one can grow directly on the
graphene layer, growing on the graphene layer was preferred because
this additional step (when using the pseudo graphene) of
exfoliating the graphene layer to leave the 6 3.times.6 3
reconstructed SiC was not needed when growing directly on the
graphene layer. Once the graphene layer was grown, growth of a SiC
layer was performed.
[0118] Both ramping in H.sub.2 and ramping in Ar were carried
out.
[0119] When samples were ramped and grown in a H.sub.2 carrier gas,
exfoliation yield was 0%, which was not desirable. When ramping in
H.sub.2 to the growth temperature for SiC growth, which was a
standard way to grow SiC on a SiC substrate, the morphology was
advantageously smooth. Smooth silicon carbide films may facilitate
patterning (e.g., patterning of surface features) on the silicon
carbide film after exfoliation and/or transfer, e.g., towards
forming devices. However, there were problems including difficulty
removing the grown SiC film, at least because the graphene had been
etched in the H.sub.2, thus making remote epitaxy impractical. This
etching resulted in a 0% yield in exfoliation of SiC films grown
with the ramping in H.sub.2. An example of surface morphology using
a growth process similar to the standard SiC growth with a ramp in
H.sub.2 is shown in FIG. 2.
[0120] FIG. 2 shows Nomarski micrographs of SiC films grown on what
was originally (graphene layer)/SiC, with a ramping to growth
temperature conducted in H.sub.2 (FIG. 2A) over an on-axis
substrate and (FIG. 2B) over a 4.degree. off-axis substrate.
Exfoliation did not take place because the graphene was etched away
during the ramp to growth temperature. FIG. 2A shows the growth
over an on-axis substrate (wafer) while FIG. 2B is for growth over
4.degree. off-axis (being c-oriented with angles cut off-axis
toward the [11-20] direction) substrate. The morphology was
improved for a SiC film grown over an off-cut substrate (e.g., FIG.
2B) compared to a SiC film grown over an on-axis substrate (e.g.,
FIG. 2A). Without wishing to be bound by theory, the on-axis grown
SiC film (e.g., FIG. 2A) had grain boundaries likely due to island
growth, but the off-axis grown SiC film (e.g., FIG. 2B) likely grew
by step flow growth.
[0121] To mitigate the etching of graphene or prevent this etching
from happening, ramping in Argon (Ar) was carried out.
Additionally, the standard flow rate (5 slm) of Ar during the ramp
to growth temperature for SiC film growth was not used as the
standard flow rate could otherwise have potentially grown
additional graphene or reduced the quality of the graphene layer.
Instead, the flow during the ramp to growth temperature was
increased 50 slm to prevent further graphene growth.
[0122] When the samples were ramped and grown in Ar and then
switched to H.sub.2 carrier gas after a short growth duration
(between or equal to 3 minutes (min) and 10 min), exfoliation yield
was increased to between or equal to 40% and 50%. When ramping to
growth temperature took place in a higher flow of Ar than the
growth of the graphene layer, SiC film morphology typically looked
similar to FIG. 3A and FIG. 3B. Growth conditions were the same for
both runs, results of which are depicted in FIG. 3. FIG. 3 shows
Nomarski micrographs of SiC films grown by remote epitaxy over
on-axis SiC substrates, with a graphene layer in between the
substrate and the growing film, with SiC films grown (FIG. 3A) for
4 min and (FIG. 3B) 1 hr. Both samples were ramped in Ar to the
desired growth temperature and then H.sub.2 was used for the growth
process instead of Ar. To achieve a smoother morphology over the
on-axis substrate, samples were ramped in 50 slm Ar to a growth
temperature of 1620.degree. C. Once the growth temperature was
reached, the carrier gas was changed from Ar to H.sub.2 and the
precursors were turned on at the same time to reduce or prevent
etching of the graphene layer. At this point, all flows were
increased from a growth rate of 1.5 .mu.m/hr to 2.5 .mu.m/hr. The
sample depicted in FIG. 3A was grown for 4 min while the sample
depicted in FIG. 3B was grown for 1 hr. The morphology improved for
the thicker material so that the boundaries seen in FIG. 3A have
been slightly reduced in FIG. 3B as the material started to grow
and coalesce. One would typically have observed, previous to this
study, at least some 3C--SiC grown over on-axis substrates, which
3C--SiC was advantageously not present in these samples.
Accordingly, it was not expected to form such a good quality
silicon carbide film on an on-axis silicon carbide substrate.
[0123] When ramped and grown in Ar and then switched to H.sub.2
after a short growth duration, and growth temperatures were lower
than for the samples typified by FIG. 3A and FIG. 3B, the
exfoliation yield remained between or equal to 40% and 50%. When
samples were ramped and grown in Ar, the exfoliation yield
advantageously increased further to between or equal to 60% and
70%. When SiC films were grown at significantly lower temperatures
(in a range of between or equal to 1350.degree. C. and 1500.degree.
C.) compared to temperatures (in a range of between or equal to
1500.degree. C. and 1800.degree. C.) for standard growth of SiC,
the morphology roughened and polycrystalline material was grown for
some cases instead of single crystalline material. An example of
this is shown in FIG. 4, where FIG. 4A was grown at 1450.degree. C.
and FIG. 4B at 1540.degree. C. FIG. 4 shows Nomarski micrographs of
samples ramped in Ar and grown in Ar for (FIG. 4A) 10 min and then
30 min in H.sub.2 at 1450.degree. C., (FIG. 4B) 1 hr at
1540.degree. C. in Ar only, (FIG. 4C) 10 min at 1400.degree. C. in
Ar only and (FIG. 4D) 10 min at 1540.degree. C. in Ar only. The
morphology advantageously had a low concentration of grain
boundaries when grown at low temperatures for short durations and
with low growth rates (1.5 microns/hour (.mu.m/hr)) as compared to
the samples grown at higher growth rates (2.5 .mu.m/hr). A growth
rate of 1.5 .mu.m/hr typically corresponded to 312 sccm of 2%
silane in H.sub.2, and 100% propane flowing at a rate corresponding
to a typical C:Si ratio of 1.55. A growth rate of 2.5 .mu.m/hr
typically corresponded to 500 sccm of 2% silane in H.sub.2, and
100% propane flowing at a rate corresponding to a typical C:Si
ratio of 1.55. In FIG. 4A, SiC was grown in 50 slm Ar for 10 min
and then switched to H.sub.2 for 30 min at a growth rate of 1.5
.mu.m/hr and temperature of 1450.degree. C. In FIG. 4B, SiC was
grown for 1 hr in 50 slm Ar, a growth temperature of 1540.degree.
C. and a growth rate of 2.5 .mu.m/hr. The higher growth temperature
improved the morphology, where the size of the grains was
significantly larger than the size of grains grown at the lower
temperature. When samples were grown in Ar for 10 min at
1400.degree. C. (FIG. 4C) and 1540.degree. C. (FIG. 4D), the
nucleation of SiC was observed. The stepped morphology was still
present. Morphologies with an advantageously low concentration of
grain boundaries were grown at the lower temperatures (see FIG. 4C
and FIG. 4D) when grown at a low growth rate (1.5 .mu.m/hr) and for
a short duration. At least for reasons discussed elsewhere herein,
it was not expected to form such a good quality silicon carbide
film (e.g., FIG. 4D) on an on-axis silicon carbide substrate.
[0124] For the sample depicted in FIG. 4A, SEM was taken (FIG. 5A)
of the sample and it was clear that there were grain boundaries
across the surface of the grown SiC film. While there were grain
boundaries, it was found with Electron Backscatter Diffraction
(EBSD) (see FIG. 5B and FIG. 5C) that this SiC film was single
polytype in both (FIG. 5B) in-plane and (FIG. 5C) out-of-plane
reflections. FIG. 5 shows (FIG. 5A) SEM image of surface
morphology, (FIG. 5B) in-plane and (FIG. 5C) out-of-plane EBSD
image and (FIG. 5D) SEM image of exfoliated SiC of a sample grown
with growth conditions from FIG. 4A. The material was
advantageously single crystalline as determined from the EBSD.
[0125] Single crystalline silicon carbide films that are free of
grain boundaries may be free of the different stresses and defects
that would otherwise be present at the grain boundaries. Devices
including single crystalline silicon carbide films may have
superior performance to devices comprising silicon carbide films
that are polycrystalline and/or have more than one polytype. A
single polytype may be desirable at least because each polytype has
different electrical properties, which may impact device
performance. If different polytypes are present in the silicon
carbide film, then the electrical performance may vary across the
film, which may not be desirable.
[0126] This sample depicted in FIG. 5 was initially grown at
1450.degree. C. for 10 min in 50 slm Ar and then grown for an
additional 30 min in H.sub.2 with a growth rate of 1.5 .mu.m/hr.
This sample was exfoliated and there was some evidence of spalling
that occurred as seen in FIG. 5D, where the image shown is the
exfoliated material. The lines in the bottom left of the image are
spalling evidence. However, exfoliation still took place even with
spalling occurring.
[0127] In the samples shown in FIG. 4 and FIG. 5 which were grown
over on-axis substrates, there was still evidence of pseudo
graphene present with a SiC film on top, as determined via high
resolution transmission electron microscopy (HRTEM), see e.g., FIG.
6. FIG. 6 is an HRTEM image (top) showing that there was pseudo
graphene present (see, e.g., bottom reference figure) after SiC
film growth by remote epitaxy. The bottom reference figure shows
pseudo graphene, which figure is adapted from Norimatsu W., et al,
"Transitional structures of the interface between graphene and
6H--SiC (0001)," Chemical Physics Letters 468 (2009) 52-56,
incorporated herein by reference. As shown in FIG. 6, there was a
dislocation 202 in the substrate at the bottom right of the HRTEM
image (top) that was not present in the SiC film grown by remote
epitaxy. This HRTEM supports that there was pseudo graphene present
after the growth of a SiC film by remote epitaxy, allowing
exfoliation of the SiC film from the substrate.
[0128] When samples were grown in Ar at 1620.degree. C., the
morphology was roughened as seen in FIG. 7 SEM images. FIG. 7 shows
SEM and EBSD of samples grown at 1620.degree. C. for 20 min in 50
slm Ar over (FIG. 7A-FIG. 7B) 4.degree. off-axis, (FIG. 7C-FIG. 7D)
2.degree. off-axis and (FIG. 7E-FIG. 7F) on-axis substrates. The
morphology was roughened for the higher offcut material (e.g., FIG.
7A) and there were mixed polytypes for the on-axis growth (e.g.,
FIG. 7F). The substrate off-cut was varied in these samples for a
comparison of SiC film remote epitaxy growth over on-axis
substrates (FIG. 7E-FIG. 7F), 2.degree. off-axis substrates (FIG.
7C-FIG. 7D), and 4.degree. off-axis substrates (FIG. 7A-FIG. 7B).
As seen (FIG. 7A and FIG. 7C), the morphology was wavy for the two
off-axis substrates, but the on-axis (FIG. 7E) had triangular
features indicating 3C--SiC. Samples depicted in FIG. 7A and FIG.
7C had a particularly favorable crystal structure; each had no
grain boundaries and consisted essentially of one polytype. Using
EBSD, it was determined that the off-axis material produced single
crystal 4H--SiC material (FIG. 7B and FIG. 7D) while the on-axis
had a mixture of 4H--SiC and 3C--SiC (FIG. 7F).
[0129] The quality of these SiC films depicted in FIG. 7 was
determined to be high, as investigated by X-ray diffraction (XRD)
rocking curves and by XRD reciprocal space maps (RSM). FIG. 8 shows
XRD rocking curves of the (002) and (004) for (FIG. 8A) 4.degree.
off-axis substrates, (FIG. 8C) 2.degree. off-axis substrates, and
(FIG. 8E) on-axis substrates and RSM of the (004) for (FIG. 8B)
4.degree. off-axis substrates, (FIG. 8D) 2.degree. off-axis
substrates, and (FIG. 8F) on-axis substrates. FIG. 8 shows results
for samples grown over 4.degree. off-axis substrates (FIG. 8A-FIG.
8B), 2.degree. off-axis substrates (FIG. 8C-FIG. 8D) and on-axis
(FIG. 8E-FIG. 8F) substrates in Ar at 1620.degree. C. The narrowest
FWHM (XRD rocking curve) sample was grown over the 4.degree.
off-axis substrate (FIG. 8A), the second narrowest was for the
on-axis (FIG. 8E) and the widest was produced over the 2.degree.
off-axis substrate (FIG. 8C). The RSMs for all of the samples (FIG.
8B, FIG. 8D, FIG. 8F) showed to have no spread in the kx (x-axis)
indicating these were high quality SiC films, along with the FWHM
being relatively narrow for the thicknesses that were grown, <2
.mu.m of SiC film.
[0130] The presence (or density) or absence of dislocations of SiC
films depicted in FIG. 7-FIG. 8 was investigated by TEM, and no
dislocations were observed. Using the process of growing in Ar for
20 min at 1620.degree. C. and looking at the sample grown over
4.degree. off-axis substrates, there were no dislocations in the
SiC film, see e.g., FIG. 9. FIG. 9 shows TEM images (left and
right) and an SEM image (center) of an SiC film grown by remote
epitaxy on a 4.degree. off-axis substrate in Ar at 1620.degree. C.
This indicated that growing a SiC film via a remote epitaxy process
reduced and/or eliminated dislocations in the SiC film, which may
greatly benefit devices.
[0131] The samples grown and described in FIG. 7-FIG. 9 were
exfoliated, and exfoliation yield was between or equal to 60% and
70%. Results of exfoliation are shown in FIG. 10, where a SiC film
was grown at 1620.degree. C. in Ar over each of 4.degree. off-axis
substrates (FIG. 10A-FIG. 10B), 2.degree. off-axis substrates (FIG.
10C-FIG. 10D), and on-axis (FIG. 10E-FIG. 10F) substrates. FIG. 10
shows photographs and SEM images of exfoliated SiC films grown by
remote epitaxy at 1620.degree. C. in Ar over (FIG. 10A-FIG. 10B)
4.degree. off-axis, (FIG. 10C-FIG. 10D) 2.degree. off-axis, and
(FIG. 10E-FIG. 10F) on-axis substrates. The photograph images for
each (FIG. 10A, FIG. 10C, FIG. 10E) show the substrate on the left
and the transferred SiC film grown by remote epitaxy on the right.
The SEM images (FIG. 10B, FIG. 10D, FIG. 10F) also show the same,
where the substrate ("sub") after exfoliation is the top SEM image
and the bottom SEM image is the exfoliated SiC film grown by remote
epitaxy ("tape"). Spalling did occur in these films when
transferred (see, e.g., spalling marks 1022 in FIG. 10B), but
transfer was still demonstrated.
[0132] From exfoliation trials, it was found that when samples were
ramped in Ar, grown for a short duration in Ar and then switched to
H.sub.2 flow for the remainder of growth, the exfoliation yield was
between or equal to 40% and 50%. When samples were ramped and grown
in Ar, the exfoliation yield increased to between or equal to 60%
and 70%.
[0133] For the growth of SiC films by remote epitaxy, a SiC film
was grown on a graphene layer that was Si sublimated from SiC, and
then the SiC film was removed and/or transferred to a desired
substrate. The process is scalable for manufacturing purposes, as
4'' (10.16 cm) wafers were exfoliated. SiC films were grown on
(graphene layer)/SiC over on-axis substrates with good morphology
and single crystallinity. Previously to this study, when growth of
SiC had taken place over on-axis SiC substrates, there had
generally been a combination of island and step flow growth, which
did not happen with this process. Along with these benefits, the
SiC films had fewer dislocations than in the substrate, as the
dislocations did not propagate into SiC films grown by remote
epitaxy. Without wishing to be bound by theory, this was also
likely the case for extended defects such as basal plane
dislocations, which would favorably impact epitaxial SiC film
quality for device applications.
[0134] A particularly useful feature to this process was ramping in
Ar to reduce or eliminate etching of a graphene layer. If one were
to grow a SiC film by ramping to a growth temperature and growing
using H.sub.2 as the carrier gas, there would have been no remote
epitaxy as most or all of the graphene layer would have been etched
away and one would just be growing a SiC film on a SiC substrate.
In order to grow a SiC film by remote epitaxy, the graphene samples
were ramped in a high flow rate of Ar and then SiC film growth
either took place in Ar, or started in Ar flow for a short duration
and then switched to H.sub.2. Otherwise, exfoliation of the grown
SiC film did not occur. When samples were ramped and grown in a
H.sub.2 carrier gas, exfoliation yield was 0%. However, when ramped
and grown in Ar and then switched to H.sub.2 after a short growth
duration, the exfoliation yield increased to between or equal to
40% and 50%. When a sample was ramped and grown in Ar, the
exfoliation yield increased further to between or equal to 60% and
70%.
[0135] While several embodiments of the present invention have been
described and illustrated herein, those of ordinary skill in the
art will readily envision a variety of other means and/or
structures for performing the functions and/or obtaining the
results and/or one or more of the advantages described herein, and
each of such variations and/or modifications is deemed to be within
the scope of the present invention. More generally, those skilled
in the art will readily appreciate that all parameters, dimensions,
materials, and configurations described herein are meant to be
exemplary and that the actual parameters, dimensions, materials,
and/or configurations will depend upon the specific application or
applications for which the teachings of the present invention
is/are used. Those skilled in the art will recognize, or be able to
ascertain using no more than routine experimentation, many
equivalents to the specific embodiments of the invention described
herein. It is, therefore, to be understood that the foregoing
embodiments are presented by way of example only and that, within
the scope of the appended claims and equivalents thereto, the
invention may be practiced otherwise than as specifically described
and claimed. The present invention is directed to each individual
feature, system, article, material, and/or method described herein.
In addition, any combination of two or more such features, systems,
articles, materials, and/or methods, if such features, systems,
articles, materials, and/or methods are not mutually inconsistent,
is included within the scope of the present invention.
[0136] The indefinite articles "a" and "an," as used herein in the
specification and in the claims, unless clearly indicated to the
contrary, should be understood to mean "at least one."
[0137] The phrase "and/or," as used herein in the specification and
in the claims, should be understood to mean "either or both" of the
elements so conjoined, i.e., elements that are conjunctively
present in some cases and disjunctively present in other cases.
Other elements may optionally be present other than the elements
specifically identified by the "and/or" clause, whether related or
unrelated to those elements specifically identified unless clearly
indicated to the contrary. Thus, as a non-limiting example, a
reference to "A and/or B," when used in conjunction with open-ended
language such as "comprising" can refer, in one embodiment, to A
without B (optionally including elements other than B); in another
embodiment, to B without A (optionally including elements other
than A); in yet another embodiment, to both A and B (optionally
including other elements); etc.
[0138] As used herein in the specification and in the claims, "or"
should be understood to have the same meaning as "and/or" as
defined above. For example, when separating items in a list, "or"
or "and/or" shall be interpreted as being inclusive, i.e., the
inclusion of at least one, but also including more than one, of a
number or list of elements, and, optionally, additional unlisted
items. Only terms clearly indicated to the contrary, such as "only
one of" or "exactly one of," or, when used in the claims,
"consisting of," will refer to the inclusion of exactly one element
of a number or list of elements. In general, the term "or" as used
herein shall only be interpreted as indicating exclusive
alternatives (i.e. "one or the other but not both") when preceded
by terms of exclusivity, such as "either," "one of," "only one of,"
or "exactly one of." "Consisting essentially of," when used in the
claims, shall have its ordinary meaning as used in the field of
patent law.
[0139] As used herein in the specification and in the claims, the
phrase "at least one," in reference to a list of one or more
elements, should be understood to mean at least one element
selected from any one or more of the elements in the list of
elements, but not necessarily including at least one of each and
every element specifically listed within the list of elements and
not excluding any combinations of elements in the list of elements.
This definition also allows that elements may optionally be present
other than the elements specifically identified within the list of
elements to which the phrase "at least one" refers, whether related
or unrelated to those elements specifically identified. Thus, as a
non-limiting example, "at least one of A and B" (or, equivalently,
"at least one of A or B," or, equivalently "at least one of A
and/or B") can refer, in one embodiment, to at least one,
optionally including more than one, A, with no B present (and
optionally including elements other than B); in another embodiment,
to at least one, optionally including more than one, B, with no A
present (and optionally including elements other than A); in yet
another embodiment, to at least one, optionally including more than
one, A, and at least one, optionally including more than one, B
(and optionally including other elements); etc.
[0140] Use of ordinal terms such as "first," "second," "third,"
etc., in the claims to modify a claim element does not by itself
connote any priority, precedence, or order of one claim element
over another or the temporal order in which acts of a method are
performed, but are used merely as labels to distinguish one claim
element having a certain name from another element having a same
name (but for use of the ordinal term) to distinguish the claim
elements.
[0141] In the claims, as well as in the specification above, all
transitional phrases such as "comprising," "including," "carrying,"
"having," "containing," "involving," "holding," and the like are to
be understood to be open-ended, i.e., to mean including but not
limited to. Only the transitional phrases "consisting of" and
"consisting essentially of" shall be closed or semi-closed
transitional phrases, respectively, as set forth in the United
States Patent Office Manual of Patent Examining Procedures, Section
2111.03.
* * * * *