U.S. patent application number 15/869021 was filed with the patent office on 2018-06-07 for growing graphene on substrates.
The applicant listed for this patent is Applied Materials, Inc.. Invention is credited to Cara BEASLEY, Majeed A. FOAD, Ralf HOFMANN.
Application Number | 20180158677 15/869021 |
Document ID | / |
Family ID | 50545237 |
Filed Date | 2018-06-07 |
United States Patent
Application |
20180158677 |
Kind Code |
A1 |
BEASLEY; Cara ; et
al. |
June 7, 2018 |
GROWING GRAPHENE ON SUBSTRATES
Abstract
Embodiments described herein provide methods and apparatus for
forming graphitic carbon such as graphene on a substrate. The
method includes providing a precursor comprising a linear
conjugated hydrocarbon, depositing a hydrocarbon layer from the
precursor on the substrate, and forming graphene from the
hydrocarbon layer by applying energy to the substrate. The
precursor may include template molecules such as polynuclear
aromatics, and may be deposited on the substrate by spinning on, by
spraying, by flowing, by dipping, or by condensing. The energy may
be applied as radiant energy, thermal energy, or plasma energy.
Inventors: |
BEASLEY; Cara; (Scotts
Valley, CA) ; HOFMANN; Ralf; (Soquel, CA) ;
FOAD; Majeed A.; (Sunnyvale, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Applied Materials, Inc. |
Santa Clara |
CA |
US |
|
|
Family ID: |
50545237 |
Appl. No.: |
15/869021 |
Filed: |
January 11, 2018 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
15427367 |
Feb 8, 2017 |
9905418 |
|
|
15869021 |
|
|
|
|
14425578 |
Mar 3, 2015 |
9595436 |
|
|
PCT/US2013/066501 |
Oct 24, 2013 |
|
|
|
15427367 |
|
|
|
|
61718258 |
Oct 25, 2012 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B05D 3/141 20130101;
H01L 21/02527 20130101; H01L 21/0237 20130101; C01B 32/184
20170801; B05D 3/065 20130101; H01L 21/02628 20130101; B05D 3/061
20130101; H01L 21/02664 20130101; B05D 5/12 20130101; H01L 21/288
20130101; H01L 21/02345 20130101; H01L 21/02376 20130101; C01B
32/186 20170801; H01L 21/02318 20130101; H01L 21/0234 20130101 |
International
Class: |
H01L 21/02 20060101
H01L021/02; H01L 21/288 20060101 H01L021/288; C01B 32/182 20170101
C01B032/182; C01B 32/184 20170101 C01B032/184 |
Claims
1. An apparatus for forming a graphene layer on a substrate, the
apparatus comprising: a deposition chamber having a substrate
support disposed therein; a liquid precursor applicator disposed in
the deposition chamber opposite the substrate support; one or more
radiant energy assemblies disposed in the chamber opposite the
substrate support; and a conduit coupled to the liquid precursor
applicator, wherein the conduit is configured to deliver a
hydrocarbon precursor and a solvent to the liquid precursor
applicator.
2. The apparatus of claim 1, wherein the substrate support is
coupled to a motor and configured to rotate.
3. The apparatus of claim 2, wherein the motor is configured to
rotate the substrate support between about 100 revolutions per
second and about 1000 revolutions per second.
4. The apparatus of claim 1 further comprising: a nozzle coupled to
the liquid precursor applicator.
5. The apparatus of claim 4, wherein the liquid precursor
applicator is disposed adjacent a central region of the substrate
support.
6. The apparatus of claim 1, wherein the one or more radiant energy
assemblies comprise a radiant energy source and a reflector.
7. The apparatus of claim 6, wherein the radiant energy source is
an ultraviolet energy source.
8. The apparatus of claim 7, wherein the ultraviolet energy source
is an ultraviolet lamp, an ultraviolet light emitting diode, an
ultraviolet laser source, or combinations thereof.
9. The apparatus of claim 8, wherein the ultraviolet laser source
is a xenon excimer laser, an argon fluorine excimer laser, or
krypton fluorine excimer laser.
10. The apparatus of claim 7, wherein the ultraviolet energy source
is configured to generate a power density of between about 1
mW/cm.sup.2 and about 1 W/cm.sup.2.
11. The apparatus of claim 1, wherein the hydrocarbon precursor is
one or more of a linear conjugated hydrocarbon, an aromatic
hydrocarbon, a cycloaliphatic hydrocarbon, derivatives thereof, and
combinations thereof.
12. The apparatus of claim 11, wherein the linear conjugated
hydrocarbon is selected from the group consisting of
1,3-pentadiene, 1,3-hexadiene, 2,4-pentadiene, 2,4-hexadiene, and
1,3,5-heptatriene.
13. The apparatus of claim 1, wherein the solvent is benzene.
14. An apparatus for forming a graphene layer on a substrate, the
apparatus comprising: a substrate handling module having a robot
disposed therein; a deposition chamber coupled to the substrate
handling module, the deposition chamber comprising: a substrate
support; a liquid precursor applicator disposed in the deposition
chamber opposite the substrate support; one or more radiant energy
assemblies disposed in the chamber opposite the substrate support;
and a conduit coupled to the liquid precursor applicator, wherein
the conduit is configured to deliver a hydrocarbon precursor and a
solvent to the liquid precursor applicator; and a cure chamber
coupled to the substrate handling module.
15. The apparatus of claim 14, wherein a motor is coupled to the
substrate support and the motor is configured to rotate the
substrate support between about 100 revolutions per second and
about 1000 revolutions per second.
16. The apparatus of claim 14 further comprising: a nozzle coupled
to the liquid precursor applicator.
17. The apparatus of claim 16, wherein the liquid precursor
applicator is disposed adjacent a central region of the substrate
support.
18. The apparatus of claim 14 further comprising: a drying chamber
coupled to the substrate handling module.
19. The apparatus of claim 14 further comprising: a pre-treatment
chamber coupled to the substrate handling module.
20. An apparatus for forming a graphene layer on a substrate, the
apparatus comprising: a substrate handling module having a robot
disposed therein; a deposition chamber coupled to the substrate
handling module, the deposition chamber comprising: a substrate
support; a liquid precursor applicator disposed in the deposition
chamber opposite the substrate support; one or more radiant energy
assemblies disposed in the chamber opposite the substrate support;
and a conduit coupled to the liquid precursor applicator, wherein
the conduit is configured to deliver a hydrocarbon precursor and a
solvent to the liquid precursor applicator; and a cure chamber
coupled to the substrate handling module, wherein the cure chamber
comprises an ultraviolet radiation source.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 15/427,367, filed Feb. 8, 2017, which is a continuation of U.S.
application Ser. No. 14/425,578 filed Mar. 3, 2015, which is a
National Stage Entry of PCT/US2013/066501 filed Oct. 24, 2013,
which claims priority from U.S. Provisional Application No.
61/718,258 filed Oct. 25, 2012. The above-mentioned applications
are hereby incorporated by reference in their entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] Embodiments of the present invention generally relate to an
apparatus and a method of growing graphene on various substrates
for different applications.
Description of the Related Art
[0003] Graphene is a flat monolayer of polycyclic carbon atoms
arranged into a quasi two-dimensional (2D) honeycomb lattice of
mostly sp.sup.2 bonds, and is a basic building block for graphitic
carbon material of other dimensionalities. Graphene can be wrapped
into 0D fullerenes, rolled into 1D nanotubes or stacked into 3D
graphites. Graphite is generally not useful for electronic devices,
whereas graphene, with its intrinsic semi-metal and zero-gap
electric properties, tunable band gap, and strong mechanical
strength (as one of the strongest materials ever tested), is a
suitable material for integrated circuit (IC) fabrication (e.g.,
for constructing quantum computers using anionic circuits).
Graphene is proposed to be used in many different applications,
such as gas sensors, nano-electronics, interconnects, transistors,
transparent conducting electrodes, ultracapacitors, solar cells,
ITO replacement, engineered piezoelectricity, diffusers,
distillation of ethanol, membrane-type devices, graphene
nano-ribbons, graphene optical modulators, graphene bio-devices,
and many others. However, graphene is currently only grown in the
lab and no efficient process for making graphene exists, making it
very expensive to grow. For example, graphene produced by
exfoliation is among the most expensive materials on Earth.
[0004] Graphene can be grown in a CVD or epitaxial process onto a
metal-containing catalyst surface (e.g., substrates with nickel or
copper on their surfaces to seed the growth of graphene) using a
gaseous carbon source that requires very high deposition
temperatures (e.g., 900.degree. C. or higher) and is difficult to
grow directly on silicon substrates, and graphene grown on metal
and transferred to non-metal substrates requires meticulous surface
attachment in many cases to achieve conductivity. Graphene has been
shown to grow on silicon carbide substrates using an epitaxial
process or a silicon evaporation process, but the temperature has
to be higher than 1,000.degree. C. Graphene may also be formed by
reduction of graphene oxide sheets at high temperatures. These
applications of growing graphene films are not suitable for most
device fabrication due to thermal budget requirements. For example,
substrates for CMOS devices typically have a temperature threshold
at about 400.degree. C.
[0005] Furthermore, even at high temperatures, current CVD or
epitaxial graphene growing processes require long deposition times,
due to low reactivity of the gaseous carbon sources used and low
efficiency in incorporating reactive species into a growing
graphene film. Further, CVD or epitaxial-deposited graphene films
are not uniform, resulting in randomly oriented grains at sizes
less than one square millimeter (mm) and varying numbers of
graphene layers. Growth is often nucleated at a number of locations
simultaneously, contributing to the formation of randomly oriented
grains in a graphene monolayer/sheet.
[0006] Assembly of the gaseous reactive species on the surface of a
substrate in a manner that is conducive to a monolayer of graphene
growth is a major challenge. The gaseous precursors participate in
a number of side reactions resulting in a loss of reactive species,
polymerization with unwanted functional groups, and undesirable
side products, all of which decrease the number of active precursor
molecules available for graphene growth. Thus, there is a need to
find a wider range of precursors and source compounds for growing
graphene.
[0007] Graphene's electric properties are strongly linked to its
thickness and length. For example, a graphene monolayer is less
than 1 nm thick, typically at about 3 angstroms, as compared to the
thickness of a semi-conductor film generally between 150 angstroms
to 5000 angstroms. Most graphitic carbon films are grown to a
thickness of 100 nm or thicker. In some such films, patches or
spots of graphene at a length of about 1-2 microns have been
observed. Such patches are not long enough for device applications,
however, and it is difficult to grow graphene that covers the whole
surface of a substrate (e.g., a silicon wafer).
[0008] Therefore, there is a need to develop a low temperature
process, to find new precursors, and to grow high quality graphene
films on a larger scale for graphene's many industrial
applications.
SUMMARY OF THE INVENTION
[0009] Embodiments described herein provide methods and apparatus
for forming graphitic carbon such as graphene on a substrate. The
method includes providing a precursor comprising a linear
conjugated hydrocarbon, depositing a hydrocarbon layer from the
precursor on the substrate, and forming graphene from the
hydrocarbon layer by applying energy to the substrate. The
precursor may include template molecules such as polynuclear
aromatics, and may be deposited on the substrate by spinning on, by
spraying, by flowing, by dipping, or by condensing. The energy may
be applied as radiant energy, thermal energy, or plasma energy.
[0010] An apparatus for practicing methods described herein
includes a chamber with a rotatable substrate support and a fluid
applicator. A source of hydrocarbon fluids is coupled to the
applicator. One or more radiant sources may be disposed in the
chamber for exposing a substrate to radiant energy, such as UV
radiation. Gas may be flowed through the chamber through an inlet
and outlet. Fluid application and UV exposure may be performed in
the same chamber or in different chambers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] So that the manner in which the above recited features of
the present invention can be understood in detail, a more
particular description of the invention, briefly summarized above,
may be had by reference to embodiments, some of which are
illustrated in the appended drawings. It is to be noted, however,
that the appended drawings illustrate only typical embodiments of
this invention and are therefore not to be considered limiting of
its scope, for the invention may admit to other equally effective
embodiments.
[0012] FIG. 1 is a flow chart summarizing a method according to one
embodiment.
[0013] FIG. 2 is a schematic side view of an apparatus according to
another embodiment.
[0014] FIG. 3A is a plan view of an apparatus according to another
embodiment.
[0015] FIG. 3B is a plan view of an apparatus according to another
embodiment.
DETAILED DESCRIPTION OF THE INVENTION
[0016] It has been found that graphene can be readily grown on a
substrate by forming a thin film of a pre-ordered hydrocarbon
precursor material that readily assembles into graphene by
application of energy. Suitable hydrocarbon precursor materials are
typically planar or quasi-planar molecules that have no significant
three-dimensional functionality. Hydrocarbon molecules that have
conjugated bonds lying substantially in one plane are useful in
this regard. Template molecules may also be used to facilitate
selective growth of graphene more than three-dimensional graphitic
carbon. The dimensionality of the carbon material may be influenced
or controlled by adjusting the amount of three-dimensional
precursors in the thin film. Graphene is produced by applying
energy to the thin film, in the form of radiant energy, thermal
energy, or plasma.
[0017] Substrates on which a graphene film may be formed using
methods described herein include semiconductor substrates such as
silicon, germanium, silicon-germanium, compound semiconductors such
as III/V and II/VI semiconductors, combinations thereof. Also
included are substrates having a dielectric surface material such
as metal oxides, ceramics, metal nitrides, semiconductor oxides
such as silicon oxides, semiconductor nitrides such as silicon
nitrides, metal and semiconductor oxynitrides, semiconductor
carbides, carbohydrides, and oxycarbohydrides, plastic, rubber, and
combinations thereof. Metals such as copper, aluminum, gold,
silver, and combinations thereof, are also included. Notably,
methods described herein may be used to form a graphene film on a
substrate having regions of different composition such as
semiconductor regions, dielectric regions, and metal regions, as
would be encountered with a typical semiconductor device substrate
having conductive and dielectric features.
[0018] FIG. 1 is a flow diagram summarizing a method 100 according
to one embodiment. A precursor solution is formed at 102. The
precursor solution has a hydrocarbon precursor and a solvent. The
hydrocarbon precursor is a compound or mixture of compounds that
readily forms a conjugated planar carbon network when exposed to
energy. Compounds suitable for such purposes are typically planar
compounds or linear compounds with no significant three-dimensional
functionality. Linear hydrocarbons such as linear paraffins, linear
olefins, and linear dienes and/or poly-enes may be used. Conjugated
dienes and poly-enes are often useful for forming a graphene sheet.
In one embodiment, 1,3 pentadiene (also known as piperylene) is
used as a graphene precursor. Other suitable precursors usable
alone or in mixtures include 1,3 butadiene, 1,3 hexadiene, 2,4
pentadiene, 2,4 hexadiene, and 1,3,5 heptatriene. Linear olefins
that may be used alone or in mixtures with other compounds include
all isomers of hexene, pentene, and butene. Aromatic molecules,
such as benzene, toluene, and xylene, and derivatives such as
styrene, vinyl benzene, and the like, may also be used. Parylene
(i.e., paraxylene monomer, dimer, or polymer) may also be used as a
precursor. Cycloaliphatic hydrocarbons such as cyclohexane and
substituted variants thereof may also be used.
[0019] Compounds useful as hydrocarbon precursors for graphene film
formation also include template compounds that facilitate or
encourage formation of a conjugated carbon monolayer. Planar
aromatic compounds such as benzene or polynuclear aromatics may be
useful in this regard. In one embodiment, pyrene is used as a
template compound. Other template compounds include naphthalene,
anthracene, phenanthrene, chrysene, perylene, coronene,
hexabenzopericoronene, benza and benzo deriviatives thereof,
partially or fully hydrogenated derivatives thereof, and
combinations thereof.
[0020] The solvent is typically a volatile compound easily removed
at relatively low temperature and having a low viscosity. The
solvent may be aprotic, for example a hydrocarbon such as pentane,
isopentane, hexane, benzene, cyclohexane, hexene, isopentene,
toluene, heptane, isohexane, heptene, isohexene, and mixtures and
derivatives thereof. The solvent may also be protic, for example an
oxygenated organic compound like ethers, alcohols, ketones,
aldehydes, or carboxylic acids. Ethanol, isopropanol, and phenol
may be used in some embodiments. Protic solvents can be helpful in
lowering the energy threshold of the graphene formation reaction.
Additionally, volatile solvents with radical affinity or hydrogen
affinity may also be useful. The solvent may also be a mixture of
compounds, protic and aprotic, which may be selected to control
preferential deposition of precursors on a substrate from
solution.
[0021] The precursor solution is applied to a substrate at 104 to
form a thin film. The thin film typically has a thickness less than
about 10 nm, such as less than about 5 nm, for example about 3 nm.
The precursor solution may be applied by any process capable of
forming a thin film, including spin-on deposition, roll-on
deposition, spray deposition, dipping, and condensation. The thin
film is typically formed at a temperature not much above room
temperature to facilitate adhesion of the precursors to the
substrate. A thin film may be formed at a temperature from about
-10.degree. C. to about 300.degree. C., such as between about
0.degree. C. and about 100.degree. C., for example about 25.degree.
C. or room temperature. Use of heavier or more viscous solvents
will typically accompany higher deposition temperatures.
[0022] The precursor solution is typically formulated to have a
viscosity of about 3.0 cP or less at room temperature to facilitate
forming a thin film on the substrate. In a spin coating process, a
film having a thickness of no more than about 5 nm may be formed by
applying a fluid having a room temperature viscosity of about 3.0
cP or less to a substrate spinning at a rate of about 1000
revolutions per second for about 30 seconds. The particular
conditions for forming a thin film by a spin-on process depend on
the precursor composition, temperature, and substrate.
[0023] If a solvent is present, the solvent typically evaporates at
least in part during the coating process. A spin coating process,
in particular, can evaporate some or all of the solvent. It is
therefore useful, in embodiments using solvents, to include enough
solvent to achieve the desired film thickness before the film
congeals. In some embodiments, solvent content and spin rate may be
selected to provide a radial flow regime that helps orient
molecules deposited on the substrate to improve the uniformity of
the resulting graphene layer. It is believed that a pre-ordered
film of the precursor molecules reduces formation of defects and
grain boundaries in the graphene layer.
[0024] The precursor mixture may also be applied to the substrate
using a dipping process. The substrate is disposed in a liquid
holding chamber which is filled to a desired level with the dipping
fluid, which may be any of the precursor compositions described
herein. The substrate may be dipped vertically, horizontally, or at
any angle between. Alternately, the precursor fluid may be applied
to the substrate by a ribbon or edge applicator at one edge of the
substrate and allowed to flow under gravity to an opposite edge of
the substrate to form a film.
[0025] The precursor mixture may feature components that provide a
degree of adhesion to the substrate, or the substrate surface may
be prepared to facilitate a desired degree of adhesion between the
substrate surface and the precursor mixture. Some adhesion of the
precursor molecules to the substrate surface may be useful in
embodiments featuring volatile components and/or elevated
temperatures to slow volatilization of the molecules while
carbon-carbon bonds are formed. In one aspect, electronegative
functionalities may be added to a portion of the hydrocarbon
precursor molecules if the surface to be coated is a hydrophillic
surface, such as a metal. Electronegative functionalities, for
example atoms having electronegativity of about 1.8 or higher
attached to a terminal carbon of the molecule may promote adhesion
of precursor molecules to the substrate. In some embodiments, a
portion of the precursor molecules may be functionalized while
other hydrocarbon molecules remain unfunctionalized. In such an
embodiment, molecular affinities between the functionalized
molecules and the non-functionalized molecules will provide a
degree of adhesion for the precursor mixture tending to reduce
unwanted volatility. Functional adherents that may be added to
hydrocarbon molecules include silicon (i.e., silanes), germanium
(i.e., germanes), boron (i.e., boranes), tin (i.e., stannanes),
phosphorus (i.e., phosphines), nitrogen (i.e., amines), arsenic
(i.e., arsines), and halogens (i.e., fluorocarbons and
chlorocarbons). Van deer Waals interactions, such as hydrophobic
interactions can also be used to promote surface adhesion and
ordering. Hydrophobic molecules tend to stick to hydrophobic
surfaces, such as pyrene on silicon, especially if the pyrene is
suspended in a polar solvent. Thus, adhesion of a hydrophobic
precursor to a hydrophobic surface may be promoted by suspending
the hydrophobic precursor in a polar solvent.
[0026] At 106, the solvent is removed from the thin film to yield a
stationary hydrocarbon film that may contain trace amounts of the
solvent in a non-fluid matrix. The solvent may be removed by
thermal volatilization at temperatures of about 150.degree. C. or
less, or at ambient or sub-ambient temperatures optionally in the
presence of a flowing atmosphere. The resulting film may be a
single molecular layer of carbon and hydrogen in a substantially
ordered arrangement of precursor molecules, or the film may have
regions of varying molecular thickness such as regions of molecular
bilayers and trilayers. It is thought that graphene is most readily
formed by a molecular monolayer of hydrocarbon, but graphene may
also result in areas of hydrocarbon bilayers and trilayers.
Graphitic carbon having a large fraction of graphene, for example
about 80% graphene or higher, with interspersed domains of glassy
carbon or graphite, may also be obtained.
[0027] When precursor molecules are pre-ordered on a substrate, it
is thought that linear molecules provide an advantage in the
ability to orient uniformly. Dienes, in particular, are thought to
orient in substantial parallelism with double bonds in a staggered
configuration, such that the double bonds of one molecule line up
beside the single bonds of a neighboring molecule. Such an
arrangement facilitates forming graphene with minimum application
of energy because pi-bonded electrons can flow into aromatic
configurations around adjacent carbon atoms as hydrogen radicals
are dislodged.
[0028] At 108, energy is applied to the thin film to convert the
film to graphitic carbon, glassy carbon, and/or graphene. The
energy may be radiant energy, thermal energy, or plasma energy.
Exemplary embodiments include UV radiation, inert gas plasma, for
example argon plasma, and heating to a temperature of at least
about 250.degree. C. UV radiation may be applied using one or more
UV lamps, a source of intense UV radiation, such as an LED source,
a UV laser, or a combination thereof. UV wavelengths between about
170 nm to about 260 nm are typically used. The UV radiation may be
applied to the entire substrate in a single exposure, or successive
portions of the substrate may be exposed, as in a laser embodiment.
The radiant energy of UV radiation applied to the substrate to
accomplish the conversion to graphene is typically between about 1
mW/cm.sup.2 and about 1 W/cm.sup.2, such as between about 20
mW/cm.sup.2 and about 50 mW/cm.sup.2, for example about 25
mW/cm.sup.2, and may be applied for a duration between about 10
seconds and about 10 minutes, for example about 1 minute.
Alternately, an intense UV source may be applied for a shorter
duration. For example pulses from an excimer laser such as a xenon,
argon fluorine, or krypton fluorine laser of duration 1 nsec to 50
nsec may be used, with suitable power, to deliver converting
radiation to successive treatment areas of the substrate.
[0029] Alternately, the substrate may be exposed to an activated
gas, such as a plasma, radical gas, or electron gas to form
graphene, graphitic carbon, or glassy carbon from the hydrocarbon
layer. A plasma or radical gas of a species or mixture that is
substantially unreactive to hydrocarbons is typically used, such as
a plasma comprising a noble gas. A remote plasma or an in-situ
plasma, such as an inductively coupled plasma, may be used,
typically with weak or no bias applied to the substrate. Exposure
to plasma may be useful in increasing grain size of graphene formed
from a pre-ordered film by catalyzing bonding across grain
boundaries.
[0030] The properties of a graphitic carbon layer, or a glassy
carbon layer, formed by methods herein may be controlled by
adjusting the precursor mixture composition, the surface
properties, and/or the film formation conditions. As noted above,
forming a hydrocarbon film having a thickness less than about 5 nm
is conducive to formation of graphene. A thicker hydrocarbon film,
such as between about 5 nm and about 20 nm, will form a glassy
carbon film having graphene domains. Such a film is typically
between about 50% graphene and about 80% graphene, with the balance
being glassy carbon domains. Such films have electrical properties
that are intermediate between those of graphene and those of glassy
carbon, depending on grain size. Such properties may also be
adjusted by providing more three-dimensional molecules in the
precursor. Molecules with significant non-planar structure provide
pathways for a carbon matrix to grow in three dimensions, rather
than only two dimensions. Thus, including precursor molecules with
significant non-planar structure encourages formation of
non-graphene domains, which may be used to control the properties
of the resulting film.
[0031] In one embodiment, a precursor solution is formed by mixing
pyrene, 1,3 pentadiene, and ethanol as a solvent. A molar ratio of
1,3 pentadiene to pyrene is typically greater than 1. As described
above, the pyrene is included as a graphene template molecule to
facilitate organization of a large-grain graphene network, so the
molar ratio of 1,3 pentadiene to pyrene may be any arbitrarily high
number, such as between about 10.sup.0 and about 10.sup.9, such as
between about 1 and about 100, for example about 50. The mixture
may have any concentration of ethanol, for example up to about 75%
ethanol, but will typically be between about 0% and about 50%
ethanol by volume, for example about 30% ethanol by volume. It
should be noted that either pyrene or 1,3 pentadiene alone, with or
without solvent, may be used to make graphene by adjusting the film
formation conditions and energy input to the conversion reaction
accordingly.
[0032] The precursor solution described above is applied to a
substrate. The ethanol content of the mixture may be adjusted to
improve coating depending on the nature of the substrate. In one
aspect, the precursor solution is applied by a spin-on process in
which the substrate is rotated at a rate of 100 rps while an
aliquot of between about 10 .mu.L to about 100 .mu.L, such as
between about 30 .mu.L and about 70 .mu.l, for example about 50
.mu.L, of the precursor mixture is dropped near the center of the
substrate. Rotation is maintained for a period of 15 sec to 3
minutes, depending on viscosity of the precursor mixture, for
example about 30 sec. Application of the precursor mixture to the
substrate is typically performed at a temperature between about
-10.degree. C. and about 200.degree. C., such as between about
0.degree. C. and about 100.degree. C., for example about 25.degree.
C. or room temperature. Depending on the substrate, the precursor
application may be performed under an inert atmosphere, such as
nitrogen, argon, or hydrogen, or a combination thereof, typically
at a nominal pressure such as atmospheric pressure, low vacuum, or
slight overpressure (e.g. between about 0.9 atm and about 1.1
atm).
[0033] In other embodiments, the precursor may be applied to the
substrate by disposing the substrate on a moving conveyor and
translating the substrate beneath a spray dispenser that covers the
substrate as it moves beneath. The precursor solution may also be
applied by dipping, pouring, flowing, condensing, or any convenient
process for wetting a substrate with a liquid.
[0034] The substrate coated with the thin film of precursor is
dried at ambient or elevated temperature of up to about 150.degree.
C. under an inert atmosphere such as nitrogen, argon, or hydrogen,
or a combination thereof, which may be stationary or moving. A
drying agent or filter may be used in some embodiments to remove
evaporated solvent from the vapor phase to accelerate drying. For
example, the substrate coated above may be dried at 40.degree. C.
for about 5 minutes. It should be noted that a substrate coated
with a thin film of hydrocarbon without use of a solvent, or even a
pure hydrocarbon, may be dried to reduce the fluid film on the
substrate to a non-fluid stationary film of molecular hydrocarbons
from one layer thick to a few layers thick, for example up to five
layers thick. Such a film may technically be a liquid, but if the
layer is thin enough for surface energy to dominate thermal energy,
the molecules will behave as if the layer is a solid, and the layer
will be a stationary non-fluid layer.
[0035] The dried substrate is exposed to UV radiation by disposing
the substrate on a support facing a UV source, such as a lamp or
bank of lamps. The substrate support may be stationary or moving.
For smaller substrates, a single UV lamp may provide radiation
sufficient to convert the thin hydrocarbon film to graphene. For
example, a single 200 or 300 mm substrate may be disposed on a
support in a box having a single 75 W UV lamp and exposed to
radiation from the lamp for 60 seconds to accomplish the
conversion. A higher power lamp, or a plurality of lamps, may also
be used and the duration adjusted accordingly. The substrate
support may also move the substrate beneath the UV lamp or lamps at
a speed commensurate with the power of the lamps to accomplish the
desired irradiation.
[0036] In one aspect, formation of graphene may be accomplished
using a staged UV exposure to increase the grain size of the
resulting graphene film. A first exposure may partially convert the
hydrocarbon film to graphene, leaving domains of graphene separated
by domains of hydrocarbon, and a second exposure may complete the
conversion to graphene or graphitic carbon. The staged exposure
typically features a rest period between exposures to allow the
film to stabilize prior to the second exposure. A staged exposure
may include any desired number of exposures, and each exposure may
feature radiation having the same power density or a different
power density. In one aspect, a first exposure may substantially
convert all hydrocarbon to graphene while subsequent exposures
increase grain sizes by activating bonding across grain boundaries.
The staged exposure may be accomplished in a single chamber or in
multiple chambers. In one aspect, the staged exposure may feature a
conversion exposure and an anneal exposure, the conversion exposure
including one or more UV treatments and the anneal exposure
including one or more UV treatments.
[0037] UV exposure for conversion to graphene is typically
performed under a chemically inert flowing atmosphere. A purge gas
is flowed through the chamber in which the substrate is disposed
such that gases emitted by the substrate during conversion to
graphene, which may include hydrogen, organic substances such as
alcohols and peroxides, or other substances if other
electronegative species are included in the pre-oriented film, may
be removed. Inert gases suitable for such service include argon (or
any noble gas), nitrogen, and hydrogen. During treatment of a 200
mm or 300 mm substrate, purge gas may be flowed through the chamber
at a rate between about 1 sLm and about 10 sLm, for example about 5
sLm. Treatment of larger substrates, up to 6 m.sup.2, may include
flowing a purge gas at rates commensurate with the larger
processing volume, for example between about 1 sLm and 100 sLm, for
example about 20 sLm. Pressure is maintained near atmospheric
pressure, and temperature is near ambient.
[0038] In another aspect, conversion to graphene may be
accomplished by exposure to other forms of energy such as thermal
energy, plasma, activated gas, or combinations thereof, which may
be accompanies by UV radiation. In a staged exposure, each exposure
may be any convenient combination of the energy source described
above.
[0039] The method 100 may be repeated, if desired, on a single
substrate to form graphene on portions of the substrate having
different properties. For example, a substrate having metal and
dielectric regions in a surface thereof may be subjected to a first
graphene formation process that selectively forms graphene on the
metal regions of the substrate, and then may be subjected to a
second graphene formation process that selectively forms graphene
on the dielectric regions of the substrate surface. In such a
treatment, the precursor mixture for the second graphene formation
process may be selected so that the precursor mixture
preferentially wets the dielectric regions of the substrate
surface, but does not wet the pre-deposited graphene film formed
over the metal areas of the substrate surface.
[0040] Prior to forming a graphene, graphitic carbon, or glassy
carbon film on a substrate, the substrate surface may be prepared
or cleaned by any convenient method. For metal portions of the
substrate, an acid wash may be used to remove surface oxide, or a
reducing environment such as hydrogen plasma may be used to remove
oxygen from the metal surface. For dielectric portions of the
substrate, a non-reactive plasma, such as a noble gas plasma (e.g.
helium, argon, neon, etc.) or a hydrogen plasma may be used to
remove impurities. For silicon portions, oxide and other impurities
may be removed by a dry clean process using fluorine chemistry such
as the SICONI.RTM. process from Applied Materials, Inc., of Santa
Clara, Calif. A wet or vapor HF treatment may also be used. For
carbon portions of the substrate, an inert plasma as described
above may be used to remove impurities from the substrate surface.
If the substrate surface features portions having different
compositions, such as metal portions and dielectric portions,
treatments compatible with all portions may be performed. For
example, an inert plasma may remove impurities from multiple
materials in the substrate surface. Also, staged cleaning and
surface preparation treatments may be performed wherein a first
chemistry operates preferentially on a first material of the
surface while a second chemistry operates preferentially on a
second material of the surface. Finally, if planarization of the
substrate surface prior to forming graphene, graphitic carbon, or
glassy carbon is desired, an inert gas plasma with an appropriate
substrate bias may be used to planarize the substrate, or a
polishing method known in the art may be used.
[0041] FIG. 2 is a schematic cross-sectional view of an apparatus
200 according to an embodiment of the invention. The apparatus 200
may be used to practice parts of, or all of, the method 100. The
apparatus 200 includes a chamber 202 with a substrate support 204
disposed therein. The substrate support 204 is connected to a motor
206 that rotates the substrate support 204, as indicated by arrow
208. Rotation of the substrate enables forming a thin film of
liquid on a substrate.
[0042] An applicator 210 extends toward a central region of the
substrate support 204, and a nozzle 222 applies a liquid precursor
to a substrate disposed on the substrate support 204 near a center
thereof. The motor 206 is capable of rotating the substrate support
204 at rates described above in connection with the method 100 of
FIG. 1. The liquid precursor applied by the applicator 210 is
sourced from a precursor source 212, which is generally a
hydrocarbon source, for example a source of 1,3 pentadiene for
practicing an embodiment of the method 100 disclosed above. The
precursor source 212 may include multiple sources of different
substances, for example a source of 1,3 pentadiene, a source of
pyrene, and a source of ethanol, with appropriate valving and
controls (not shown) for blending a precursor mixture. The
applicator 210 is connected to the precursor source 212 by a
conduit 214.
[0043] The apparatus 200 may have one or more radiant energy
assemblies 216, each radiant assembly 216 including a radiant
source 218 and a waveguide 220, which may be a reflector. The
radiant sources 218 may include thermal sources and UV sources. In
one embodiment, a single radiant energy assembly 216 is a UV lamp
assembly. Power is naturally coupled to the radiant sources 216,
and is not shown in FIG. 2. The radiant sources 216 may be
controlled to irradiate a substrate on the substrate support 204
according to any of the embodiments of the method 100, as described
above.
[0044] Gases may be provided to the chamber 202 by a gas inlet 224,
and may be removed by a gas outlet 226. If desired, the substrate
support 204 may include thermal control features, as is known in
the art.
[0045] In another embodiment, an apparatus for forming a graphene
film may have two chambers, each of which performs part of the
graphene formation process. In one embodiment, a deposition chamber
forms a thin pre-ordered hydrocarbon precursor film on a substrate,
while a cure chamber exposes the film to energy to form graphene.
If desired, the two chambers may be coupled to a front-end module
that moves substrates into and out of the two chambers. FIGS. 3A
and 3B are plan views of two embodiments 300 and 350. In the
embodiment 300 of FIG. 3A, a first chamber 302 may be a deposition
chamber for forming a hydrocarbon film on a substrate, and a second
304 may be a cure chamber. A substrate handling module 306 is
coupled to the first chamber 302 and the second chamber 304 to load
and unload substrates from the two chambers. The substrate handling
module 306 may have a robot 308 to manipulate substrates.
[0046] In the embodiment 350 of FIG. 3B, a first chamber 352 is
included, which may be a substrate pre-treatment chamber, for
example a cleaning chamber, which may be a wet clean or a dry clean
chamber, as described above. Alternately, the first chamber 352 may
be a deposition chamber. A second chamber 354 may be a deposition
chamber, if the first chamber 352 is a pre-treatment chamber, or a
drying chamber, if the first chamber 352 is a deposition chamber.
In either of the embodiments 300 and 350, a staging module may be
disposed between the substrate handling module 306 and the
chambers, if desired. The staging module may be useful to control
substrate throughput, and in some embodiments the staging module
may serve as a load-lock for vacuum processes.
[0047] While the foregoing is directed to embodiments of the
present invention, other and further embodiments of the invention
may be devised without departing from the basic scope thereof, and
the scope thereof is determined by the claims that follow.
* * * * *