U.S. patent application number 11/416724 was filed with the patent office on 2010-08-05 for method for making carbon nanotube-base device.
This patent application is currently assigned to Tsinghua University. Invention is credited to Shou-Shan Fan, Liang Liu.
Application Number | 20100193350 11/416724 |
Document ID | / |
Family ID | 37655989 |
Filed Date | 2010-08-05 |
United States Patent
Application |
20100193350 |
Kind Code |
A1 |
Liu; Liang ; et al. |
August 5, 2010 |
METHOD FOR MAKING CARBON NANOTUBE-BASE DEVICE
Abstract
A method for making a carbon nanotube-based device is provided.
A substrate having a shadow mask layer to define an unmasked
surface area thereon is provided. A sputter source is disposed on
the shadow mask layer. The sputter source is configured for
supplying a catalyst material and depositing the catalyst material
onto the substrate. A catalyst layer including at least one
catalyst block is formed on the substrate. A thickness of the at
least one catalyst block is gradually decreased from one end to
another opposite end thereof. The at least one catalyst block has a
region with a thickness proximal or equal to an optimum thickness.
A carbon source gas is introduced. At least one carbon nanotube
array extending from the catalyst layer using a chemical vapor
deposition process is formed. The at least one carbon nanotube
array is arc-shaped, and bend in a direction of deviating from the
region.
Inventors: |
Liu; Liang; (Beijing,
CN) ; Fan; Shou-Shan; (Beijing, CN) |
Correspondence
Address: |
PCE INDUSTRY, INC.;ATT. Steven Reiss
288 SOUTH MAYO AVENUE
CITY OF INDUSTRY
CA
91789
US
|
Assignee: |
Tsinghua University
Beijing City
CN
HON HAI Precision Industry CO., LTD.
Tu-Cheng City
TW
|
Family ID: |
37655989 |
Appl. No.: |
11/416724 |
Filed: |
May 3, 2006 |
Current U.S.
Class: |
204/192.15 ;
427/249.1; 977/847 |
Current CPC
Class: |
B82Y 30/00 20130101;
Y10S 977/84 20130101; B82Y 40/00 20130101; Y10S 977/844 20130101;
Y10S 977/843 20130101; B82B 3/00 20130101; C01B 32/162 20170801;
Y10S 977/842 20130101; C01B 2202/08 20130101 |
Class at
Publication: |
204/192.15 ;
427/249.1; 977/847 |
International
Class: |
C23C 14/34 20060101
C23C014/34; C23C 16/00 20060101 C23C016/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 22, 2005 |
CN |
200510036148.1 |
Claims
1. A method for making a carbon nanotube-based device, the method
comprising the steps of: providing a substrate having a shadow mask
layer formed thereon, the shadow mask layer being configured for
defining an unmasked surface area on the substrate; disposing a
sputter source on the shadow mask layer, the sputter source being
configured for supplying a catalyst material and depositing the
catalyst material onto the substrate; forming a catalyst layer
comprising at least one catalyst block on the unmasked surface area
of the substrate, a thickness of the at least one catalyst block
being gradually decreased from a first end thereof to an opposite
second end thereof; the at least one catalyst block having a region
with a thickness proximal or equal to an optimum thickness at which
carbon nanotubes growing fastest, a thicker region having a
thickness that is thicker than the optimum thickness, and a thinner
region having a thickness that is thinner than the optimum
thickness, such that the carbon nanotubes in the thicker and
thinner regions grow slower than the carbon nanotubes at the
region; introducing a carbon source gas; and forming at least one
carbon nanotube array extending from the catalyst layer using a
chemical vapor deposition process, wherein the at least one carbon
nanotube array being arc-shaped, and bending in a direction of
deviating from the region.
2. The method of claim 1, wherein the step of forming the catalyst
layer comprising the sub-steps of: sputter-depositing a catalyst
material on the substrate to form an initial catalyst layer;
removing the shadow mask layer from the substrate; and patterning
the initial catalyst layer to form the catalyst layer.
3. The method of claim 2, further comprising a step of marking the
region of the initial catalyst layer, before patterning the initial
catalyst layer.
4. The method of claim 2, wherein the sputter source comprising a
sputter target made from the catalyst material, the sputter target
being selected from the group consisting of a surface sputter
target and a linear sputter target.
5. The method of claim 2, wherein the catalyst material is selected
from the group consisting of iron, cobalt, nickel, and alloys
thereof.
6. The method of claim 2, wherein the shadow mask layer has a
plurality of sidewalls substantially perpendicular to the
substrate.
7. The method of claim 2, wherein a thickness of a position of the
initial catalyst layer distant from the shadow mask layer with a
distance of .lamda. satisfies the condition:
T(.lamda.)=T.sub.0/2.times.(1+.lamda./ {square root over
(.lamda..sup.2+h.sup.2)}) where, the T(.lamda.) is a thickness of
the position of the initial catalyst layer distant from the shadow
mask layer with a distance of .lamda., T.sub.0 is a thickness of
the position under a situation that no shadow mask layer formed on
the substrate during the sputter-deposition process; h is a height
of the shadow mask layer.
8. The method of the claim 1, wherein a thickness of the first end
of the at least one catalyst block is in the range from 5 to 20
nanometers; a thickness of the second end of the at least one
catalyst block is in the range from 1 to 10 nanometers.
9. (canceled)
10. The method of claim 1, wherein the step of forming the catalyst
layer comprising the sub-steps of: sputter-depositing a catalyst
material on the substrate to form the catalyst layer; and removing
the shadow mask layer from the substrate.
11. The method of claim 1, further comprising a step of annealing
the substrate with the catalyst layer in an oxygen-containing
environment to form nano-sized catalyst oxide particles before
introducing the carbon source gas.
12. The method of claim 1, wherein the substrate is made from a
material selected from the group consisting of silicon, glass, and
metal.
13. (canceled)
14. The method of claim 1, wherein the step of forming at least one
carbon nanotube array extending from the catalyst layer using a
chemical vapor deposition process further comprises forming at
least two carbon nanotube arrays bending in two different
directions of deviating from the region.
15. The method of claim 2, wherein the step of patterning the
initial catalyst layer to form the catalyst layer further comprises
patterning the initial catalyst layer to form a plurality of first
catalyst blocks and second catalyst blocks, a thinnest end of the
first catalyst blocks comprises of a thickness approximately equal
to the optimum thickness, and a thickest end of the second catalyst
blocks comprises of a thickness approximately equal to the optimum
thickness.
16. The method of claim 15, wherein the plurality of first catalyst
blocks and second catalyst blocks is patterned to be staggeringly
positioned at opposite sides of the region with the thickness
proximal or equal to the optimum thickness.
17. The method of claim 1, wherein a height of the shadow mask
layer is less than mean free path of a catalyst atom generated from
the sputter source under a predetermined sputter-deposition
process, wherein the mean free path S of a catalyst atom satisfies
the condition: S = kT 2 .pi. d 2 p ##EQU00002## where d is a
diameter of the catalyst atom, p is an operating pressure of the
sputter-deposition process, k is the Boltzmann constant
(1.38066.times.10.sup.-23 J/K (Joule per Kelvin)), T is an
operating absolute temperature of the sputter-deposition
process.
18. The method of claim 1, wherein a distance from the sputter
source to the substrate is greater than the mean free path of the
catalyst atom generated from the sputter source under the
predetermined sputter-deposition process.
19. The method of claim 6, wherein the catalyst layer is formed
around the shadow mask layer.
20. The method of claim 1, wherein the catalyst material is iron,
the carbon source gas is ethylene, a temperature at which the
carbon nanotubes are grown is about 700.degree. C., the optimum
thickness of the catalyst layer is about 5 nanometers.
21. A method for making a carbon nanotube-based device, the method
comprising the steps of: forming a catalyst layer comprising a
gradient thickness being gradually decreased from a first end of
the catalyst layer to an opposite second end of the catalyst layer,
wherein the catalyst layer comprises a region with a thickness
proximal or equal to an optimum thickness at which carbon nanotubes
grow fastest; patterning the catalyst layer to form a plurality of
first catalyst blocks and second catalyst blocks staggeringly
positioned at opposite sides of the region with the thickness
proximal or equal to the optimum thickness, wherein a thinnest end
of the first catalyst blocks comprises of a thickness approximately
equal to the optimum thickness, and a thickest end of the second
catalyst blocks comprises of a thickness approximately equal to the
optimum thickness; introducing a carbon source gas; and forming a
plurality of carbon nanotube arrays extending from the first
catalyst blocks and second catalyst blocks using a chemical vapor
deposition process, wherein the carbon nanotube arrays extending
from the first catalyst blocks bend toward a thickest end of the
first catalyst blocks, and the carbon nanotube arrays extending
from the second catalyst blocks bend toward a thinnest end of the
second catalyst blocks.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is related to one copending U.S. patent
application entitled "METHOD FOR MAKING CARBON NANOTUBE-BASED
DEVICE", concurrently filed here with (Docket No. US7938), and
having the same assignee as the instant application. The disclosure
of the above-indentified application is incorporated herein by
reference.
TECHNICAL FIELD
[0002] The present invention generally relates to a method for
making a carbon nanotube-based device.
BACKGROUND
[0003] Carbon nanotubes are very small tube-shaped structures
essentially having the composition of a graphite sheet, formed as a
tube. Carbon nanotubes produced by arc discharge between graphite
rods were first discovered and reported in an article by Sumio
Iijima entitled "Helical Microtubules of Graphitic Carbon" (Nature,
Vol. 354, Nov. 7, 1991, pp. 56-58). Carbon nanotubes have very good
electrical conductance due to their structure. They are also
chemically stable, and have very small diameters (less than 100
nanometers) and large aspect ratios (length/diameter). Due to these
and other properties, it has been suggested that carbon nanotubes
can play an important role in fields such as microscopic
electronics, materials science, biology and chemistry.
[0004] Although carbon nanotubes promise to have a wide range of
applications, better control is needed over the building and
organization of nanotube-based architectures. Normally, the
orientation of growing nanotubes is controlled such that the
nanotubes are rectilinear and parallel to each other. Chemical
vapor deposition has been used to produce nanotubes vertically
aligned on catalyst-printed substrates.
[0005] There have been reports of growth of aligned carbon
nanotubes using chemical vapor deposition, for instance, Z. F. Ren
et al. entitled "Synthesis of large arrays of well-aligned carbon
nanotubes on glass" (Science, Vol. 282, Nov. 6, 1998, pp.
1105-1107), S. S. Fan et al. entitled "Self-oriented regular arrays
of carbon nanotubes and their field emission properties" (Science,
Vol. 283, Jan. 22, 1999, pp. 512-514), B. Q. Wei et al. entitled
"Organized assembly of carbon nanotubes" (Nature, Vol. 416, Apr. 4,
2002, pp. 495-496), Yoon-Taek Jang et al. entitled "Lateral growth
of aligned mutilwalled carbon nanotubes under electric field"
(Solid State Communications, Vol. 126, 2003, pp. 305-308), and
Ki-Hong Lee et al. entitled "Control of growth orientation for
carbon nanotubes" (Appl. Phys. Lett., Vol. 82, Jan. 20, 2003, pp.
448-450).
[0006] However, carbon nanotubes obtained by the above-mentioned
methods are aligned along a linear direction, and/or extend
perpendicularly from the substrates. Furthermore, the method of
using an external field such as an electric field or a magnetic
field, to control a direction of growth of the carbon nanotubes is
difficult to apply in generating localized complicated structures
with plural orientations of the carbon nanotubes. Accordingly, the
range of diversity of different kinds of carbon nanotube-based
devices is limited.
[0007] What is needed is to provide a method for making a carbon
nanotube-based device with plural orientations of carbon
nanotubes.
SUMMARY
[0008] In a preferred embodiment, a method for making a carbon
nanotube-based device is provided. The method includes the
following steps of: providing a substrate having a shadow mask
layer formed thereon, the shadow mask layer being configured for
defining an unmasked surface area on the substrate; disposing a
sputter source on the shadow mask layer, the sputter source being
configured for supplying a catalyst material and depositing the
catalyst material onto the substrate; forming a catalyst layer
including at least one catalyst block on the unmasked surface area
of the substrate, a thickness of the at least one catalyst block
being gradually decreased from a first end thereof to an opposite
second end thereof, and the at least one catalyst block having a
region with a thickness proximal or equal to an optimum thickness
at which carbon nanotubes growing fastest; introducing a carbon
source gas; and forming at least one carbon nanotube array
extending from the catalyst layer using a chemical vapor deposition
process, wherein the at least one carbon nanotube array being
arc-shaped, and bending in a direction of deviating from the
region.
[0009] Theoretically, the growth rate of carbon nanotubes is
associated with a thickness of the catalyst layer used to grow them
on. Under certain conditions for growing carbon nanotubes by a
chemical vapor deposition process, the carbon nanotubes grow
fastest when the catalyst layer has a certain optimum thickness. In
particular, when the thickness of the catalyst layer is greater
than the optimum thickness, the thicker the catalyst layer, the
slower the growth rate of carbon nanotubes; when the thickness of
the catalyst layer is less than the optimum thickness, the thinner
the catalyst layer, the slower the growth rate of carbon nanotubes.
If the thickness of the catalyst layer is deliberately controlled,
so that the thickness gradually varies from a first end to an
opposite second end, and somewhere the catalyst layer has a
thickness proximal or equal to the optimum thickness for growing
carbon nanotube; additionally, carbon nanotubes have inherently
strong Van der Waals force interactions therebetween. Thereby, a
carbon nanotube-based device with plural orientations of carbon
nanotubes is obtainable.
[0010] Other advantages and novel features will become more
apparent from the following detailed description of embodiments
when taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The components in the drawings are not necessarily to scale,
the emphasis instead being placed upon clearly illustrating the
principles of the present method for making a carbon nanotube-based
device. Moreover, in the drawings, like reference numerals
designate corresponding parts throughout the several views.
[0012] FIG. 1 is a schematic, cross-sectional view of one stage in
a procedure for sputter-depositing a catalyst layer on a substrate
in accordance with the first embodiment;
[0013] FIG. 2 is a schematic, cross-sectional view of a subsequent
stage in a procedure for sputter-depositing a catalyst layer on the
substrate of FIG. 1;
[0014] FIG. 3 is an isometric view of the substrate of FIG. 2 with
the catalyst layer formed thereon, after the shadow mask layer
being removed, and a region with a thickness proximal or equal to
an optimum thickness at which carbon nanotubes growing fastest, of
the catalyst layer being marked;
[0015] FIG. 4 is similar to FIG. 3, but showing the catalyst layer
patterned into a plurality of catalyst blocks positioned at
opposite sides of the region;
[0016] FIG. 5 is an enlarged, side view of the substrate and
catalyst blocks of FIG. 4 after being annealed;
[0017] FIG. 6 is an enlarged, side view of a carbon nanotube-based
device in accordance with the first embodiment, obtained by
treating the catalyst blocks of FIG. 5;
[0018] FIG. 7 is an isometric view of the carbon nanotube-based
device of FIG. 6;
[0019] FIG. 8 is a schematic, cross-sectional view of one stage in
a procedure for sputter-depositing a catalyst layer on a substrate
in accordance with the second embodiment;
[0020] FIG. 9 is a schematic, cross-sectional view of a subsequent
stage in a procedure for sputter-depositing a catalyst layer on a
substrate of FIG. 8;
[0021] FIG. 10 is a schematic, cross-sectional view of the
substrate of FIG. 9 with the catalyst layer formed thereon, after
the shadow mask layer being removed, and a region with a thickness
proximal or equal to an optimum thickness at which carbon nanotubes
growing fastest, of the catalyst layer being marked;
[0022] FIG. 11 is an isometric view of the substrate of FIG. 10
with the catalyst layer formed thereon;
[0023] FIG. 12 is similar to FIG. 11, but showing the catalyst
layer patterned into a plurality of catalyst blocks positioned at
opposite sides of the region;
[0024] FIG. 13 is an enlarged, side view of the substrate and
catalyst blocks of FIG. 12 after being annealed;
[0025] FIG. 14 is an enlarged, side view of a carbon nanotube-based
device in accordance with the second embodiment, obtained by
treating the catalyst blocks of the substrate of FIG. 13; and
[0026] FIG. 15 is an isometric view of the carbon nanotube-based
device of FIG. 14.
[0027] The exemplifications set out herein illustrate at least one
preferred embodiment, in one form, and such exemplifications are
not to be construed as limiting the scope of the present method for
making a carbon nanotube-base device in any manner.
DETAILED DESCRIPTION OF THE EMBODIMENTS
Embodiment 1
[0028] FIGS. 1-4 together illustrate successive stages in a process
for forming a catalyst layer on a substrate, in accordance with the
first embodiment.
[0029] Referring to FIG. 1, a substrate 10 is first provided.
Suitable substrate materials include a variety of materials,
including metals, semiconductors and insulators such as silicon
(Si), glass and metal sheets. It is possible that the substrate 10
will, in practice, be a portion of a device, e.g., a silicon-based
integrated circuit device, on which nanotube formation is
desired.
[0030] A shadow mask layer 40 is formed on the substrate 10 and
located at one end portion thereof to define an unmasked surface
area 12 of the substrate 10 to be exposed. The shadow mask layer 40
is usually made from photo-resist, metal, metallic oxide, or
metallic nitride. The shadow mask layer 40 has a suitable height
(as denoted by h in FIG. 1) to shade a sputter source 20 disposed
above the shadow mask layer 40, in order to sputter-deposit a
catalyst layer having a gradient thickness on the substrate 10.
Advantageously, the shadow mask layer 40 has a plurality of
sidewalls 42 substantially perpendicular to the substrate 10. The
height of the shadow mask layer 40 is usually less than mean free
path of a catalyst atom generated from the sputter source 20 under
a predetermined sputter-deposition process. The mean free path S of
a catalyst atom satisfies the condition (1):
S = kT 2 .pi. d 2 p ( 1 ) ##EQU00001##
where, d is a diameter of the catalyst atom, p is an operating
pressure of the sputter-deposition process, k is the Boltzmann
constant (1.38066.times.10.sup.-23 J/K (Joule per kelvin)), T is an
operating absolute temperature of the sputter-deposition process.
For example, when the T.apprxeq.273.15 kelvin, p=1 pascal; the mean
free path S of a catalyst iron atom is approximately equal to
13.5.times.10.sup.-3 meters.
[0031] The sputter source 20 is configured for supplying a catalyst
material and depositing the catalyst material onto the substrate
10, so as to form a catalyst layer having a gradient thickness via
a sputter-deposition process. The catalyst material is usually
selected from iron (Fe), cobalt (Co), nickel (Ni), or alloys
thereof. Generally, a distance from the sputter source 20 to the
substrate 10 is greater than the mean free path of a catalyst atom
generated from the sputter source 20 under a predetermined
sputter-deposition process. The sputter source 20 usually includes
a surface sputter target made from a catalyst material, or a linear
sputter target made from a catalyst material. In the case of the
sputter source 20 having a linear sputter target, an optimum
solution is that a reciprocating movement of the substrate 10
relative to the sputter source 20 along a direction perpendicular
to the linear sputter target is implemented. In the illustrated
embodiment, the sputter source 20 has a surface sputter target.
[0032] Referring to FIG. 2, due to the shading effect of the shadow
mask layer 40 to the catalyst material generated from the catalyst
source 20, a catalyst layer 30 having a gradient thickness is
formed on the unmasked surface area 12 of the substrate 10 via a
sputter-deposition process. The catalyst layer 30 is located at one
side of the shadow mask layer 40. The catalyst layer 30 has a
region with a thickness proximal or equal to, an optimum thickness
at which carbon nanotubes growing fastest. A thickness of the
catalyst layer 30 is gradually varied from one end thereof to
another opposite end thereof
[0033] Generally, a thickness T(.lamda.) of a position of the
catalyst layer 30 distant from the shadow mask layer 40 with a
distance of .lamda. approximately satisfies the following condition
(2):
T(.lamda.)=T.sub.0/2.times.(1+.lamda./ {square root over
(.lamda..sup.2+h.sup.2)}) (2)
where, T.sub.0 is a thickness of the position under a situation
that no shadow mask layer 40 exists on the substrate 10 during the
sputter-deposition process; h is a height of the shadow mask layer
40.
[0034] It is realized from the condition (2) that an obvious
gradient thickness exists in the region where .lamda. varies from 0
to 2h. The size of the region is correlated with the height of the
shadow mask layer 40. For example, when the height h of the shadow
mask layer 40 is in the range from 0.1 micrometers to 10
millimeters; correspondingly, the length (starting from the shadow
mask layer 40) of the region having obvious gradient thickness is
in the range from 0.2 micrometers to 20 millimeters In practice,
the region having obvious gradient thickness usually fully covers a
region used for growing carbon nanotubes of an expected carbon
nanotube-base device.
[0035] Referring to FIG. 3, the shadow mask layer 40 is removed
from the substrate 10. A region 32 of the catalyst layer 30 is
marked for purpose of determining the growth direction of carbon
nanotubes. That is, under certain conditions for growing carbon
nanotubes by a chemical vapor deposition process, the carbon
nanotubes grow fastest at where a region of the catalyst layer 30
has an optimum thickness. If the conditions for growing the carbon
nanotubes by the chemical vapor deposition process are
predetermined, the optimum thickness can be determined accordingly.
In the illustrated embodiment, as an example, the catalyst material
is iron, a carbon source gas is ethylene, a temperature at which
the carbon nanotubes are grown is about 700.degree. C. (degrees
Celsius). Accordingly, an optimum thickness of the catalyst layer
30 for growing carbon nanotubes is about 5 nm, i.e., the region 32
in this condition has a thickness proximal or equal to 5
nanometers.
[0036] Referring to FIG. 4, the catalyst layer 30 can be patterned
to meet various configurations of resultant carbon nanotube-based
devices. The patterned catalyst layer 30 includes at least one
catalyst block having a gradient thickness. In particular, a
thickness of the at least one catalyst block gradually varies from
a first end thereof to an opposite second end thereof The least one
catalyst block somewhere has a thickness proximal or equal to the
optimum thickness for growing carbon nanotubes.
[0037] Advantageously, when the patterned catalyst layer 30 is made
from iron, a thickness of the thickest end of each of the catalyst
blocks is in the range from 5 nm to 20 nm, and a thickness of the
thinnest end of each the catalyst blocks is in the range from 1 nm
to 10 nm. In the illustrated embodiment, the patterned catalyst
layer 30 includes a plurality of catalyst blocks 33, 34
staggeringly positioned at opposite sides of the region of optimum
thickness 32. A thinnest end of each of the catalyst blocks 34 has
a thickness approximately equal to the optimum thickness, and a
thickest end of each of the catalyst blocks 33 has a thickness
approximately equal to the optimum thickness. The pattern is
defined using a photolithography process.
[0038] It is understood that, a catalyst layer having a
predetermined pattern can be directly formed, without the
patterning step as above-mentioned. The formation of such a
catalyst layer is actually the product of a series of substeps. A
shadow mask layer having a reverse pattern corresponding to the
predetermined pattern is formed on the substrate 10, to define an
unmasked surface area same to the pretermined pattern. After a
catalyst layer sputter-deposition process similar to the process as
above-mentioned being implemented, and the shadow mask layer being
removed using a lift-off process, a catalyst layer having the
predetermined pattern can be directly obtained.
[0039] FIGS. 5-6 together illustrate successive stages in a process
for forming a carbon nanotube-based device with plural orientations
of carbon nanotubes based on the above-described catalyst layer 30,
in accordance with the first embodiment.
[0040] Referring to FIG. 5, the substrate 10 with the catalyst
blocks 33, 34 is annealed in an oxygen-containing environment at
about 300.degree. C., thereby oxidizing the catalyst blocks 33, 34
to form nano-sized catalyst oxide particles 33', 34'. Consequently,
the thinner a portion of the catalyst blocks 33, 34 is, the smaller
the diameters of the catalyst oxide particles 33', 34' formed from
that portion are. Likewise, the thicker a portion of the catalyst
blocks 33, 34 is, the larger the diameters of the catalyst oxide
particles 33', 34' formed from that portion are.
[0041] Subsequently, the treated substrate 10 is placed in a
furnace (not shown), a carbon source gas is continuously introduced
into the furnace, and then a chemical vapor deposition process
similar to that of defining the region 32 of the catalyst layer 30
is implemented. In particular, the carbon source gas with a
protective gas together are continuously introduced into the
furnace at a predetermined temperature (e.g. 500-900.degree. C.).
The carbon source gas can be acetylene, ethylene, methane or any
suitable carbon-containing gas. The protective gas can be a noble
gas or nitrogen. The protective gas and carbon source gas are
introduced at suitable flow rates respectively (e.g. 160 sccm and
80 sccm respectively).
[0042] Referring to FIG. 6, a plurality of carbon nanotube arrays
50, 51 extending from the substrate 10 can be formed. During the
process of growing the carbon nanotube arrays 50, 51, the carbon
source gas is decomposed into carbon atoms and hydrogen gas in a
catalytic reaction process catalyzed by the nano-sized catalyst
oxide particles 33', 34'. Thus the catalyst oxide particles 33',
34' are deoxidized to catalyst particles 33'', 34'' by the hydrogen
gas. More detailed information on growth of a carbon nanotube array
is taught in U.S. Pat. No. 6,232,706 entitled "Self-Oriented
Bundles of Carbon Nanotubes and Method of Making Same," which is
incorporated herein by reference. Due to inherently strong Van der
Waals force interactions between the carbon nanotubes, the carbon
nanotubes are bundled together, and the carbon nanotube arrays 50,
51 extend in arc shapes bending in directions deviating from the
region of the optimum thickness 32.
[0043] Referring to FIG. 7, a resultant carbon nanotube-based
device 100 with plural orientations of the carbon nanotube arrays
50, 51 can be formed. The carbon nanotube-based device 100 includes
the substrate 10, and a plurality of carbon nanotube arrays 50, 51
extending from the catalyst layer 30, supported by the substrate
10. The carbon nanotube arrays 50, 51 are arc-shaped, and bend in
respective directions deviating from the region 32. In the
illustrated embodiment, because the catalyst layer 30 only formed
at one side of the shadow mask layer 40, so all the carbon nanotube
arrays 50, 51 of the carbon nanotube-based device 100 totally
extend along two different directions (as shown in FIG. 6).
Embodiment 2
[0044] FIGS. 8-12 together illustrate successive stages in a
process for forming a catalyst layer on a substrate, in accordance
with the second embodiment.
[0045] Referring to FIG. 8, a substrate 70 is first provided.
Suitable substrate materials include a variety of materials,
including metals, semiconductors and insulators such as silicon
(Si), glass and metal sheets. It is possible that the substrate 70
will, in practice, be a portion of a device, e.g., a silicon-based
integrated circuit device, on which nanotube formation is
desired.
[0046] A shadow mask layer 80 is formed on the substrate 70 and
located at a middle portion thereof, to define an unmasked surface
area 72 of the substrate 70 to be exposed. The shadow mask layer 80
is usually made from photo-resist, metal, metallic oxide, or
metallic nitride. The shadow mask layer 80 has a suitable height
(as denoted by h in FIG. 7) to shade a sputter source 60 disposed
on the shadow mask layer 80, in order to sputter-deposit a catalyst
layer having a gradient thickness on the substrate 70.
Advantageously, the shadow mask layer 80 has a plurality of
sidewalls 82 substantially perpendicular to the substrate 70. The
height of the shadow mask layer 80 is usually less than mean free
path of a catalyst atom generated from the sputter source 60 under
a predetermined sputter-deposition process. The mean free path (S)
of a catalyst atom satisfies the condition (1) as the first
embodiment described.
[0047] The sputter source 60 is configured for supplying a catalyst
material and depositing the catalyst material onto the substrate
70, so as to form a catalyst layer having a gradient thickness via
a sputter-deposition process. The catalyst material is usually
selected from iron (Fe), cobalt (Co), nickel (Ni), or alloys
thereof. Generally, a distance from the sputter source 60 to the
substrate 70 is greater than the mean free path of a catalyst atom
generated from the sputter source 60 under a predetermined
sputter-deposition process. The sputter source 60 usually includes
a surface sputter target made from a catalyst material, or a linear
sputter target made from a catalyst material. In the case of the
sputter source 60 having a linear sputter target, an optimum
solution is that a reciprocating movement of the substrate 70
relative to the sputter source 60 along a direction perpendicular
to the linear sputter target is implemented. In the illustrated
embodiment, the sputter source 60 has a surface sputter target.
[0048] Referring to FIG. 9, a catalyst layer 90 having a gradient
thickness is formed on the unmasked surface area 72 of the
substrate 70, via a sputter-deposition process. The catalyst layer
90 is located around the shadow mask layer 80, and has a region
with a thickness proximal or equal to an optimum thickness at which
carbon nanotubes growing fastest. A thickness of the catalyst layer
90 is gradually varied from one end thereof to another opposite end
thereof.
[0049] Generally, a thickness T(.lamda.) of a position of the
catalyst layer 90 distant from the shadow mask layer 80 with a
distance of .lamda. approximately satisfies the condition (2) as
the above-described. It is realized from the condition (2) that an
obvious gradient thickness exists in the region where .lamda.
varies from 0 to 2h; and the size of the region is correlated with
the height of the shadow mask layer 80. In practice, the region
having obvious gradient thickness usually fully covers a region
used for growing carbon nanotubes of an expected carbon
nanotube-base device.
[0050] Referring to FIGS. 10 and 11, the shadow mask layer 80 is
removed from the substrate 70. A region 92 of the catalyst layer 90
is marked for purpose of determining the growth direction of carbon
nanotubes. A method for defining the region 92 of the catalyst
layer 90 is similar to that as the first embodiment described.
[0051] Referring to FIG. 12, the catalyst layer 90 can be patterned
to meet the various configurations of resultant carbon
nanotube-based devices. The patterned catalyst layer 90 includes at
least one catalyst block having a gradient thickness. In
particular, a thickness of the at least one catalyst block
gradually varies from a first end thereof to an opposite second end
thereof. The least one catalyst block somewhere has a thickness
proximal or equal to the optimum thickness for growing carbon
nanotubes.
[0052] In the illustrated embodiment, the patterned catalyst layer
90 includes a plurality of catalyst blocks 93, 94, 95, 96 each
positioned at a side of the region 92. A thinnest end of each of
the catalyst blocks 93, 96 has a thickness proximal or equal to the
optimum thickness, and a thickest end of each of the catalyst
blocks 94, 95 has a thickness proximal or equal to the optimum
thickness. The pattern is defined using a photolithography
process.
[0053] It is understood that, a catalyst layer having a
predetermined pattern can be directly formed, without the
patterning step as above-mentioned. The formation of such a
catalyst layer is actually the product of a series of substeps. A
shadow mask layer having a reverse pattern corresponding to the
predetermined pattern is formed on the substrate 90, to define a
unmasked surface area same to the predetermined pattern. After a
catalyst layer sputter-deposition process similar to the process as
above-mentioned being implemented, and the shadow mask layer being
removed using a lift-off process, a catalyst layer having the
predetermined pattern can be directly obtained.
[0054] FIGS. 13-14 together illustrate successive stages in a
process for forming a carbon nanotube-based device with plural
orientations of carbon nanotubes based on the above-described
catalyst layer 90, in accordance with the second embodiment.
[0055] Referring to FIG. 13, the substrate 70 with the catalyst
blocks 93, 94, 95, 96 is annealed in an oxygen-containing
environment at about 300.degree. C., thereby oxidizing the catalyst
blocks 93, 94, 95, 96 to form nano-sized catalyst oxide particles
93', 94', 95', 96'. Consequently, the thinner a portion of the
catalyst blocks 93, 94, 95, 96 is, the smaller the diameters of the
catalyst oxide particles 33', 34' formed from that portion are.
Likewise, the thicker a portion of the catalyst blocks 93, 94, 95,
96 is, the larger the diameters of the catalyst oxide particles
93', 94', 95', 96' formed from that portion are.
[0056] Subsequently, the treated substrate 70 is placed in a
furnace (not shown), a carbon source gas is introduced into the
furnace, and then a chemical vapor deposition process similar to
that of defining the region 92 of the catalyst layer 90 is
implemented.
[0057] Referring to FIG. 14, a plurality of carbon nanotube arrays
101, 102, 103, 104 extending from the substrate 70 can be formed.
During the process of growing the carbon nanotube arrays 101, 102,
103, 104, the carbon source gas is decomposed into carbon atoms and
hydrogen gas in a catalytic reaction process catalyzed by the
nano-sized catalyst oxide particles 93', 94', 95', 96'. Thus the
catalyst oxide particles 93', 94', 95', 96' are deoxidized to
catalyst particles 93'', 94'', 95'', 96'' by the hydrogen gas. Due
to inherently strong Van der Waals force interactions between the
carbon nanotubes, the carbon nanotubes are bundled together, and
the carbon nanotube arrays 101, 102, 103, 104 extend in arc shapes
bending in respective directions of deviating from the region
92.
[0058] Referring to FIG. 15, a resultant carbon nanotube-based
device 1000 with plural orientations of the carbon nanotube arrays
101, 102, 103, 104 can be formed. The carbon nanotube-based device
1000 includes the substrate 70, and a plurality of carbon nanotube
arrays 101, 102, 103, 104 extending from the catalyst layer 90 (as
shown in FIG. 14), supported by the substrate 70. The carbon
nanotube arrays 101, 102, 103, 104 are arc-shaped, and bend in
respective directions deviating from the region 92. In the
illustrated embodiment, because the catalyst layer 90 formed around
the shadow mask layer 80, so all the carbon nanotube arrays 101,
102, 103, 104 of the carbon nanotube-based device 1000 totally can
extend along four different directions (as shown in FIG. 15).
[0059] It is believed that the present embodiments and their
advantages will be understood from the foregoing description, and
it will be apparent that various changes may be made thereto
without departing from the spirit and scope of the invention or
sacrificing all of its material advantages, the examples
hereinbefore described merely being preferred or exemplary
embodiments of the invention.
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