U.S. patent application number 17/248938 was filed with the patent office on 2021-08-05 for fabrication of high aspect ratio tall free standing posts using carbon-nanotube (cnt) templated microfabrication.
The applicant listed for this patent is BRIGHAM YOUNG UNIVERSITY. Invention is credited to Guohai Chen, Robert C. Davis, Richard Vanfleet.
Application Number | 20210239644 17/248938 |
Document ID | / |
Family ID | 1000005527031 |
Filed Date | 2021-08-05 |
United States Patent
Application |
20210239644 |
Kind Code |
A1 |
Chen; Guohai ; et
al. |
August 5, 2021 |
FABRICATION OF HIGH ASPECT RATIO TALL FREE STANDING POSTS USING
CARBON-NANOTUBE (CNT) TEMPLATED MICROFABRICATION
Abstract
In a general aspect, an apparatus can include a substrate and a
post disposed on the substrate. The post can include a plurality of
nanotubes and extend substantially vertically from the substrate.
The post can have an aspect ratio of a height of the post to a
diameter of the post of greater than or equal to 25:1.
Inventors: |
Chen; Guohai; (Provo,
UT) ; Davis; Robert C.; (Provo, UT) ;
Vanfleet; Richard; (Provo, UT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BRIGHAM YOUNG UNIVERSITY |
Provo |
UT |
US |
|
|
Family ID: |
1000005527031 |
Appl. No.: |
17/248938 |
Filed: |
February 12, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15299386 |
Oct 20, 2016 |
10921279 |
|
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17248938 |
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62244145 |
Oct 20, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B82Y 15/00 20130101;
B81C 1/00111 20130101; G01N 27/3278 20130101; B81C 99/0085
20130101; C01B 32/16 20170801; B01L 3/5088 20130101; G01N 27/3277
20130101; B82Y 40/00 20130101; Y10S 977/893 20130101; Y10S 977/92
20130101; Y10S 977/742 20130101 |
International
Class: |
G01N 27/327 20060101
G01N027/327; C01B 32/16 20060101 C01B032/16; B81C 1/00 20060101
B81C001/00; B81C 99/00 20060101 B81C099/00 |
Claims
1. An apparatus comprising: a substrate; and a post disposed on the
substrate, the post including a plurality of nanotubes and
extending substantially vertically from the substrate, the post
having an aspect ratio of a height of the post to a diameter of the
post of greater than or equal to 25:1.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of, and claims priority
to, U.S. patent application Ser. No. 15/299,386, filed on Oct. 20,
2016, entitled "FABRICATION OF HIGH ASPECT RATIO TALL FREE STANDING
POSTS USING CARBON-NANOTUBE (CNT) TEMPLATED MICROFABRICATION,"
which is a Non-provisional of, and claims priority to, U.S.
Provisional Patent Application No. 62/244,145, filed on Oct. 20,
2015, entitled "Fabrication of High Aspect Ratio Tall Free Standing
Posts Using Carbon-Nanotube (CNT) Templated Microfabrication", both
of which are incorporated by reference herein in their
entireties.
TECHNICAL FIELD
[0002] This description relates to the fabrication of high aspect
ratio tall free standing posts using carbon-nanotube (CNT)
templated microfabrication with subsequent plasma etching.
SUMMARY
[0003] In a general aspect, an apparatus can include a substrate
and a post disposed on the substrate. The post can include a
plurality of nanotubes and extend substantially vertically from the
substrate. The post can have an aspect ratio of a height of the
post to a diameter of the post of greater than or equal to
25:1.
[0004] Implementations can include one more of the following
features. For example, the diameter of the post can be in a range
of 5 micrometers (.mu.m) to 100 .mu.m. The post can be
substantially cylindrical. The height of the post can be greater
than or equal to 1 millimeter (mm).
[0005] The plurality of nanotubes can include a plurality of carbon
nanotubes (CNTs). At least a portion of the plurality of nanotubes
of the post can be infiltrated with carbon (C). At least a portion
of the plurality of nanotubes of the post can be infiltrated with
at least one of silicon (Si) and silicon nitride (SiN). At least a
portion of the plurality of nanotubes of the post can be plated
with a metal.
[0006] The post can be a first post and the apparatus can include a
second post disposed on the substrate and laterally spaced from the
first post, the second post including a plurality of nanotubes and
extending substantially vertically from the substrate, the second
post having an aspect ratio of a height of the second post to a
diameter of the second post of greater than or equal to 25:1.
[0007] The substrate can include a silicon (Si) wafer having an
aluminum oxide (Al.sub.2O.sub.3) layer disposed thereon. The
substrate can include a glass substrate or a metal substrate.
[0008] In another general aspect, a method can include providing a
substrate and forming a patterned catalyst layer on the substrate.
The patterned catalyst layer can define a template for carbon
nanotube growth. The template can define a pattern for formation of
a first carbon nanotube post, a second carbon nanotube post and a
supporting structure disposed between the first carbon nanotube
post and the second carbon nanotube post. The method can further
include growing carbon nanotubes on the patterned catalyst layer to
form the first carbon nanotube post, the second carbon nanotube
post and the supporting structure. The first carbon nanotube post
and the second carbon nanotube post can each have an aspect ratio
of a height to a diameter of greater than or equal to 25:1. The
supporting structure can have an aspect ratio of a height to a
width of greater than or equal to 200:1. The method can further
include removing the supporting structure, such that each of the
first carbon nanotube post and the second carbon nanotube post are
freestanding and extend substantially vertically from the
substrate.
[0009] Implementations can include one more of the following
features. For example, prior to removal of the supporting
structure, a height of the first carbon nanotube post, a height of
the second carbon nanotube post and a height of the supporting
structure can be substantially a same height. The same height can
be greater than or equal to 1 millimeter (mm).
[0010] The method can include infiltrating the first carbon
nanotube post and the second carbon nanotube post with carbon (C).
The method can include infiltrating the first carbon nanotube post
and the second carbon nanotube post with at least one of silicon
(Si) and silicon nitride (SiN). The method can include plating the
first carbon nanotube post and the second carbon nanotube post with
a metal.
[0011] Removing the supporting structure can include performing a
non-directional plasma etch to remove an upper portion of the
supporting structure, such that a lower portion of the supporting
structure remains, the lower portion of the supporting structure
being disposed on the substrate; infiltrating the first carbon
nanotube post, the second carbon nanotube post and the lower
portion of the supporting structure with carbon (C); and performing
a directional plasma etch to remove the lower portion of the
supporting structure.
[0012] The substrate can include a Si wafer having an
Al.sub.2O.sub.3 layer disposed thereon. The patterned catalyst
layer can be formed on the Al.sub.2O.sub.3 layer. Forming the
patterned catalyst layer can include forming, using
photolithography, a patterned iron (Fe) layer, the Al.sub.2O.sub.3
layer preventing diffusion of the patterned Fe layer into the Si
wafer. The substrate can include a metal substrate or a glass
substrate.
[0013] In another general aspect, an apparatus can include a
substrate and an array of carbon nanotube posts disposed on the
substrate. Each carbon nanotube post of the array of carbon
nanotube posts can include a plurality of nanotubes and can extend
substantially vertically from the substrate. Each carbon nanotube
post of the array of carbon nanotube posts can have an aspect ratio
of a height of the post to a diameter of the post of greater than
or equal to 25:1.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a diagram that schematically illustrates a method
for producing high aspect ratio carbon nanotubes (CNTs), according
to an implementation.
[0015] FIGS. 2A through 2G are cross-sections illustrating a
processing flow for producing high aspect ratio CNTs, according to
an implementation.
[0016] FIG. 3 is a diagram illustrating growth kinetic results for
producing tall CNT features, on the order of millimeters, according
to an implementation.
[0017] FIGS. 4A through 4C are images that illustrate top views of
high aspect ratio CNT structures formed using supporting
structures, according to implementations.
[0018] FIGS. 5A through 5C are images that illustrate perspective
views of tall CNT structures formed using supporting structures,
according to implementations.
[0019] FIGS. 5D through 5F are images that illustrate tall CNT
structures formed without the use of supporting structures.
[0020] FIGS. 6A through 6D are images that illustrate side views of
tall CNT structures formed using supporting structures, according
to implementations.
[0021] FIG. 7 is a diagram that illustrates a method for removal of
supporting structures from an array of high aspect ratio CNT
structures, according to an implementation.
[0022] FIGS. 8A and 8B are images that illustrate aspects related
to removal of supporting structures from an array of high aspect
ratio CNT structures, according to implementations.
[0023] FIGS. 9A through 9E are images that illustrate free standing
CNT posts after removal of supporting structures between the posts,
according to implementations.
[0024] FIGS. 10A through 10D are scanning electron microscope (SEM)
images that illustrate side-views of free standing CNT posts after
removing the CNT supporting structures, according to
implementations.
[0025] FIG. 11 is a diagram that illustrates transfer of free
standing mm-tall CNT posts to, for example, a metal pin for further
characterization, according to an implementation.
[0026] FIGS. 12A and 12B are graphs illustrating aspects of using
mm-tall CNT posts in biochemical applications, according to an
implementation.
[0027] Like reference symbols in the various drawings may indicate
like and/or similar elements.
DETAILED DESCRIPTION
[0028] The fabrication of high aspect ratio tall free standing
posts (e.g., post arrays) can be produced using Carbon Nanotube
(CNT) templated microfabrication with a sacrificial supporting
structure. Carbon-nanotube-templated microfabrication (CNT-M) can
be used to form precise high aspect ratio features in, for example,
interconnected geometries. The feature of fabrication of isolated
posts can be challenging, however. In some implementations, a CNT
shape, when grown in, for example, forests (e.g., a dense
collection of CNT structures), can be used to produce, for example,
three dimensional (3D) complex structures. In some implementations,
if those structures are relatively tall and relatively thin (have a
high aspect ratio), the structures can lose at least some aspects
of the important vertical nature of the growth (e.g., bend, break,
tip, etc.). The implementations described herein can enable the
retention of the vertical structure for very tall features. In some
implementations, CNT members (e.g., needles) of, for example,
.about.10-40 micrometers (.mu.m) in diameter can be grown to, for
example, millimeter heights. In implementations, such CNT members
can have a range of spacing between them (e.g., lateral spacing on
the substrate), such as 100-400 .mu.m center-to-center spacing.
Depending on the implementation (and the particular CNT structures
being produced) the spacing could be less than 100 .mu.m
center-to-center or greater than 400 .mu.m center-to-center.
[0029] A process is described herein that includes fabrication of
CNT posts connected by supporting structures using, for example,
CNT-M techniques followed by plasma etching (e.g., oxygen plasma
etching) to remove the sacrificial supporting structures. The
fabrication of posts can be achieved with diameters from, for
example, 10-40 um and heights up to, for example, 1.3 mm using
sacrificial supporting structures of, for example, 1-5 um in width,
with spacing such as those discussed above. With the CNT template,
isolated free standing posts from a variety of materials can be
made. For example, silicon or silicon nitride posts can be
fabricated by infiltration with silicon or silicon nitride. In some
implementations, the creation of hybrid carbon/metal (copper,
nickel) posts can also be realized through pulse
electroplating.
[0030] Low aspect ratio carbon nanotube (CNT) posts (also can be
referred to as members) can be produced using direct growth
techniques. However, as noted above, direct growth of high aspect
ratio CNT posts can be challenging, or impossible to achieve. High
aspect ratio CNT posts formed using direct growth techniques can
bend in random directions (break, tip, etc.) as they grow tall
depending on the aspect ratio. Examples of such occurrences are
shown in FIGS. 5D-5F. Free standing CNT post arrays can be
important for developing CNTs devices and for integrating them with
conventional microelectronics in many research fields.
[0031] FIG. 1 is a diagram that schematically illustrates a method
for producing high aspect ratio carbon nanotubes (CNTs), according
to an implementation. FIG. 1 illustrates the addition of supporting
structures (supporting hedges, etc.) 120 to support the growth of
CNT posts 100 in a CNT-M structure. As shown, in FIG. 1 the CNT
posts 110 and supporting structures 120 can be formed on a
substrate 100. After forming the CNT-M structure including the CNT
posts 110 and the supporting structures 120, processing 130 can be
performed to remove the supporting structures 120, resulting in an
array of free standing, high aspect ratio (e.g., with a height to
diameter ratio greater than or equal to 25:1) CNT posts 110
disposed on the substrate 100. The free standing CNT posts can be 1
millimeter (mm) or greater in height. While example approaches for
producing structures such as the, free-standing CNT posts 110 shown
in FIG. 1, the following briefly describes aspects of such
processes.
[0032] In some implementations, straight (substantially straight,
vertical, substantially vertical, and so forth) millimeter-tall CNT
posts (e.g., CNT posts 110) with diameters of, for example, 10-40
.mu.m or 5-100 .mu.m can be grown using supporting CNT structures
(e.g. supporting structures 120). For instance, CNT posts can be
grown where a vertical line through a center of a cross-section of
one end of the CNT post would intersect a cross-section take at an
opposite end of the CNT post. While the CNT posts 110 are
illustrated as being cylindrical in cross-section, in other
implementations CNT posts (or other tall, vertical CNT structures)
having other cross-sectional geometries (e.g., square, triangle,
hexagon, or other shape) can be produced using the approaches
described herein. Further, the final geometries of a given CNT
structure can affected by the removal (e.g., etching) of associated
supporting structures.
[0033] In some implementations, such free standing mm-tall CNT
posts can be produced by removing the sacrificial supporting CNT
structures by, for example, a combination of non-directional plasma
etching, carbon infiltration, and followed directional plasma
etching, such as illustrated by and described with respect to FIG.
7.
[0034] In some implementations, a sacrificial structure or
connector, such as the supporting structures 120, can be used to
link pattern features (CNT posts) together during growth of a CNT
pattern. In some implementations, the linked features can, during
growth, be constrained by their connection to remain in a vertical
(substantially vertical) orientation. In some implementations, this
sacrificial feature can then be removed, leaving the desired
feature(s), such as CNT posts, behind.
[0035] In some implementations, the supporting structure features
can be sufficiently robust to constrain the growth (supporting
structure features with widths as small as 1 um can be sufficient)
and can be removed without damaging the final intended
structure.
[0036] In some implementations, straight millimeter-tall CNT post
arrays, such as shown in FIG. 1, can be directly grown with a
sacrificial CNT supporting structure. In such implementations, a
process that includes non-directional plasma etching, carbon
infiltration, and directional plasma etching can be used to remove
the sacrificial CNT structures. Accordingly, the fabrication of
high aspect ratio free standing straight CNT post arrays can be
realized.
[0037] In some implementations, such nanotube posts can be nanotube
posts that include nanotubes including materials other than carbon.
For instance, a layer of silicon could be deposited on a CNT post
(or array of CNT posts). After depositing the silicon layer, the
CNT post(s) could be oxidized to convert the deposited silicon to
silicon dioxide (SiO.sub.2 or silica) and also convert the carbon
of the CNTs to carbon dioxide (which can be vent out of an
oxidation furnace), resulting in a nanotube post (or posts) that
include silica nanotubes.
[0038] FIGS. 2A through 2G are cross-sections illustrating a
processing flow for producing high aspect ratio CNTs, according to
an implementation. In some implementations, as shown in FIG. 2A, a
substrate can be prepared. The substrate can include, for example,
a Si wafer 200 having a buffer layer 210 disposed (formed,
deposited, etc.) thereon. In this implementation, the buffer layer
210, which can be an aluminum oxide (Al.sub.2O.sub.3) layer, can
prevent diffusion of a catalyst for CNT growth from diffusing into
the Si wafer 200. In other implementations, other substrate
materials can be used, such as metal or glass substrates.
[0039] As shown in FIGS. 2B-2E, photolithography can then be
performed to form a patterned catalyst layer 230, which can be an
iron (Fe) layer. As shown in FIG. 2B, a photoresist layer 220 can
be formed on the buffer layer 210. The photoresist layer can then
be patterned as shown in FIG. 2C to define areas of the buffer
layer 210 where the patterned catalyst layer 230 will be formed. In
an implementation, the patterned catalyst layer 230 can have a
pattern such as the pattern of CNT posts 110 and supporting
structures 120 shown in FIG. 1. In other implementations, other
patterns can be formed. As shown in FIG. 2D a catalyst material
(e.g., Fe) can be deposited and, as illustrated by FIG. 2E, an
lift-off process can be performed to remove unwanted catalyst
material and underlying photoresist 220.
[0040] As shown in FIG. 2E, chemical vapor deposition (CVD) can be
performed to grow CNT posts 110 with supporting structures 120. In
an example implementation, CNT growth can be performed in, for
example, a tube furnace. In such an approach, the structure shown
in FIG. 2E can be placed in the furnace and hydrogen gas can flow
over the sample as the furnace is heated to a desired temperature
(750 C in one implementation). This hydrogen flow can reduce any
oxide that may have formed on the catalyst layer 230 (and prevent
formation of additional oxide). After the temperature of the
furnace arrives at the desired temperature, carbon nanotubes (e.g.,
substantially vertical carbon nanotubes) can be grown by adding a
flow of ethylene gas to the hydrogen flow for a given growth time,
where the growth time depends on the desired height (aspect ratio)
of the CNT structures being formed. Carbon nanotube growth can then
be terminated by replacing the flows of hydrogen and ethylene with
an inert gas flow (e.g., an argon flow) to flush the furnace
chamber and halt carbon nanotube growth.
[0041] After CNT growth to form the CNT posts 110 and the
interconnecting supporting structures 120, the supporting
structures 120 can be removed, as shown in FIG. 2G. In an
implementation, the supporting structures 120 can be removed used a
combination of non-directional and directional plasma etching, such
in the process described below with respect to FIG. 7.
[0042] FIG. 3 is a diagram illustrating growth kinetic results
(e.g., using the growth process described above) for producing tall
CNT features, on the order of millimeters in height, according to
an implementation. FIG. 3 includes scanning electron microscope
(SEM) images 310, 320, 330 and 340 of CNT features at various
growth times, as are respectively indicated with references to the
graph for each image.
[0043] FIGS. 4A through 4C are SEM images 410, 420 and 430 that
illustrate top views of high aspect ratio CNT structures formed
using supporting structures at various magnifications, according to
implementations. As shown, in some implementations, CNT posts (with
supporting structures) that are well-ordered (e.g., regularly
spaced/arranged, substantially vertical, etc.) can be achieved
using the approaches described herein.
[0044] FIGS. 5A through 5C are SEM images 510, 520 and 530 (at
various magnifications) that illustrate perspective views of tall
CNT structures formed using supporting structures, according to
implementations. In comparison, FIGS. 5D through 5F are SEM images
540, 550 and 560 that illustrate tall CNT structures formed without
the use of supporting structures. As shown in FIGS. 5D through 5F,
such CNT structures bend in random directions, break, tip, etc.
[0045] FIGS. 6A through 6D are SEM images 610, 620, 630 and 640 (at
various magnifications) that illustrate side views of tall CNT
structures formed using supporting structures, according to
implementations. These side view SEM images 610-640 illustrate
straight (substantially straight, vertical, substantially vertical,
etc.) mm-tall CNT posts producing using the approaches described
herein.
[0046] In some implementations, removal of the sacrificial
structure can be performed by plasma etching or some other form of
removal. For example, FIG. 7 is a diagram that illustrates a
processing method 700 for removal of supporting structures from an
array of high aspect ratio CNT structures, according to an
implementation. The combination of processes in the method 700 can
be used to efficiently remove supporting structure with
reproducible results.
[0047] In FIG. 7, the upper images of each processing operation
illustrate a schematic view of the associated processing operation,
while the lower images illustrate CNT structures during the
associated processing operation. In the method 700, a two-step etch
can be performed with a carbon infiltration process performed in
between the two etch steps, where the carbon infiltration process
can be used to strengthen the CNT structures.
[0048] As shown in FIG. 7 (after growth of the CNTs posts 110 and
supporting structures 210), an initial etch process 710 can be
performed. In an implementation, the etch 710 can be performed on a
CNT structure 712 in, for example, a chamber 714 (e.g., a PYREX
chamber), and can be performed using a non-directional (oxygen)
plasma 716. In some implementations, the etch process 710 may
remove (or substantially only remove) an upper portion of the
supporting structures 120, and only remove the supporting
structures 120 a few hundred micrometers down from the top of the
supporting structures 120. A variety of conditions can be used to
perform the etch process 710. These conditions can include RF power
(e.g., in a range of 6-18 watts (W)) and air flow rate (e.g., 0-20
standard cubic centimeters per minute (sccm)). In the etch process
710, lower RF powers can increase processing time, while higher air
flow rates can destroy alignment of a CNT post array structure. In
an example implementation, the etch 710 can be performed using an
RF power of 18 W and air flow rate less than 1 sccm.
[0049] In the method 700, after the etch 710, carbon infiltration
720 can be performed, which can link the CNTs together with carbon
724 to strengthen the final structures against tipping, breaking,
etc. During carbon infiltration 720, the CNT structures 712 can be
heated 722 to various temperatures (e.g., in a range 850-900 C).
Further, carbon infiltration 720 can be performed using different
reaction gas flow rates (e.g., with a gas flow of C.sub.2H.sub.2
and H.sub.2) and different infiltration times. In some
implementations, an amount carbon 724 that is infiltrated in the
CNT structures can affect the amount of time needed to remove the
carbon infiltrated supporting structures 120. In an example
implementation, carbon infiltration 720 can be performed for 1.5
minutes at 900.degree. C. or lower.
[0050] As shown in FIG. 7, after the carbon infiltration 720, a
directional (oxygen) plasma etch process 730 can then be performed
(e.g., in a plasma etch chamber 732) on the CNT structures 712 to
remove the bottom (lower) portion of the sacrificial structure 120
(not removed by the etch process 710) using a directional plasma
734. In some implementations, the directional plasma etch process
730 can have improved performance at the bottom of the confined CNT
features (structures) as compared to the non-directional plasma
etch process 710. Different conditions can be used (varied) to
perform the directional plasma etch process 730. These conditions
can include a plasma power (e.g., in a range of 100-250 W), an
oxygen flow rate and a pressure in the etch chamber. In an
implementation, the directional plasma etch process 730 can be
performed at a power of 150 W, an oxygen flow rate of 10 sccm, and
a pressure of 500 mTorr.
[0051] In some implementations, the combination of a
non-directional etch targeting the top down of supporting
structures and a directional etch targeting the bottom up of
supporting structure can be used on a wide variety of feature
geometries.
[0052] In some implementations, a structure (e.g., a final
structure) can be etched by the processes of the method 700, which
can cause a reduction in dimensions of the final structure. Such
reductions in dimension can be accounted for in an initial design
of a desired (final) CNT structure (e.g., CNT post).
[0053] FIGS. 8A and 8B are SEM images 810 and 820 that illustrate
aspects related to removal of supporting structures from an array
of high aspect ratio CNT structures. The SEM image 810 in FIG. 8A
illustrates a CNT structure where only non-directional plasma
etching was used (as compared to the approach of FIG. 7). As shown
in FIG. 8A, only upper portions of the supporting structures were
removed after, for example, 5-10 hours of non-directional plasma
etching. The SEM image 820 in FIG. 8B illustrates a CNT structure
where only directional plasma etching was used (as compared to the
approach of FIG. 7). As shown in FIG. 8B, using only directional
plasma, CNT posts were damaged and/or lost vertical alignment.
[0054] FIGS. 9A through 9E are SEM images 910, 920, 930, 940 and
950 that illustrate free standing CNT posts after removal of
supporting structures between the posts (e.g. using the processing
method 700 of FIG. 7), according to implementations. The SEM images
910-930 of FIGS. 9A-9C illustrate an array of CNT posts at various
magnifications. The SEM images 940 and 950 of FIGS. 9D and 9E
illustrate magnified images of a single CNT post. For instance,
FIG. 9D shows a top of a CNT single post. FIG. 9E shows a bottom of
a single CNT post, illustrating that a CNT supporting structures
(such as those described herein) are removed with only some
residue.
[0055] FIGS. 10A through 10D are SEM images 1010, 1020, 1030 and
1040 that illustrate (at various magnifications) side-views of free
standing CNT posts (such as the CNT posts of FIGS. 9A-9E) after
removing associated CNT supporting structures, according to
implementations.
[0056] FIG. 11 is a diagram that illustrates transfer of free
standing mm-tall CNT posts to, for example, a metal pin for further
characterization, according to an implementation. In FIG. 11, an
image 1110 illustrates CNT posts that have been removed from a
substrate on which they were formed. An image 1120 in FIG. 11
illustrates a single CNT post (e.g., shown in the highlighted
portion of the image 1120) attached to a metal pin. In FIG. 11, an
image 1120a shows a magnified view of the CNT post attached to the
metal pin and an image 1120b shows a portion of the CNT post that
is highlighted in the image 1120a. The single CNT post shown in
FIG. 11, or an array of CNT posts, can be used in a number of
applications, such as for chemical sensing, use as neural probes,
and so forth. Depending on the particular implementation, a CNT
post, or array of CNT posts can be infiltrated with one or more
materials. For example, CNT posts can be infiltrated with carbon,
silicon, silicon nitride and/or plated with a metal, such as Ni or
Cu.
[0057] In an implementation, a CNT post array (such as those
illustrated herein) can be used as a neural probe, for example, to
detect dopamine. Such an implementation can aid in the production
of medical implants used to improve the quality of life for people
suffering from health conditions such as heart disease and
neurological disorders. Such approaches could also be implemented
using a single CNT post. FIGS. 12A and 12B are graphs illustrating
use of a single CNT post (e.g. the CNT post of FIG. 11) as a probe
for the detection of dopamine. In this example, a single CNT post
(probe) was used to detected dopamine in concentrations from 10
micromoles to 1 millimole using fast scan rate (400 V/s) cyclic
voltammetry.
[0058] As noted above, the results shown in FIGS. 12A and 12B were
achieved using a single CNT post (e.g., such as shown in FIG. 11).
In this implementation, the individual CNT post was affixed to a
metal (e.g., tungsten) pin using a silver conducting paste. The
metal pin and affixed CNT post were then coated with a layer of
wax, which was followed by dipping a portion of the CNT post in
xylene to expose that portion of the CNT post. This exposed part of
the CNT post (probe) served as an electrode to perform
electrochemical tests to detect the presence of dopamine.
[0059] FIG. 12A is a graph 1210 that illustrates current versus
time curves indicating detection of dopamine, according to an
implementation. FIG. 12B is graph 1220 that illustrates cyclic
voltammetry curves for dopamine concentration s of 1, 0.1 and 0.01
millimoles in a phosphate-buffered saline (PBS) solution at a pH of
7.4. In the cyclic voltammetry test (illustrated by FIG. 12B), a
voltage sweep from -0.6 V to 1.3 V was applied to the electrode
(CNT probe) with a 1 Hz frequency and 400 V/s scan rate. Initially,
a background current (shown in FIG. 12A from time 0 until dopamine
introduction) was measured for a flow of PBS solution. Then, as
shown in FIG. 12, dopamine started to flow (at approximately 90
seconds) and the electrode detected the presence of dopamine as a
change in current due to the electrochemical reaction of dopamine
in the PBS solution. As shown in FIG. 12A, the current increases as
the concentration of dopamine increases relative to the
concentration of the PBS solution. As the PBS solution is
completely replaced by dopamine solution, the current plateaus, as
illustrated in FIG. 12A, which also shows the gradual increase of
the current for each dopamine concentration, and also demonstrates
that the respective final current levels relate to the dopamine
concentration.
[0060] FIG. 12B shows cyclic voltammetry curves used for dopamine
detection (e.g., as shown in FIG. 12A) using the CNT probe
described above at a scan rate of 400 V/s. In FIG. 12B, an
oxidation peak of dopamine occurs at a potential of approximately
0.8 V. FIGS. 12A and 12B also illustrate that peak current is
related to dopamine concentration. Using such an approach,
measurement of dopamine concentration (or concentration of other
substances) can be accomplished using CNT post (single posts or
arrays) produced using the approaches described herein.
[0061] Further implementations are summarized in the following
examples:
[0062] Example 1: An apparatus comprising: a substrate; and a post
disposed on the substrate, the post including a plurality of
nanotubes and extending substantially vertically from the
substrate, the post having an aspect ratio of a height of the post
to a diameter of the post of greater than or equal to 25:1.
[0063] Example 2: The apparatus of example 1, wherein the diameter
of the post is in a range of 5 micrometers (.mu.m) to 100
.mu.m.
[0064] Example 3: The apparatus of example 1 or 2, wherein the post
is substantially cylindrical.
[0065] Example 4: The apparatus of one of examples 1 to 3, wherein
the height of the post is greater than or equal to 1 millimeter
(mm).
[0066] Example 5: The apparatus of one of examples 1 to 4, wherein
the plurality of nanotubes includes a plurality of carbon nanotubes
(CNTs).
[0067] Example 6: The apparatus of one of examples 1 to 5, wherein
at least a portion of the plurality of nanotubes of the post are
infiltrated with carbon (C).
[0068] Example 7: The apparatus of one of examples 1 to 6, wherein
at least a portion of the plurality of nanotubes of the post are
infiltrated with at least one of silicon (Si) and silicon nitride
(SiN).
[0069] Example 8: The apparatus of one of examples 1 to 7, wherein
at least a portion of the plurality of nanotubes of the post are
plated with a metal.
[0070] Example 9: The apparatus of one of examples 1 to 8, wherein
the post is a first post, the apparatus further comprising a second
post disposed on the substrate and laterally spaced from the first
post, the second post including a plurality of nanotubes and
extending substantially vertically from the substrate, the second
post having an aspect ratio of a height of the second post to a
diameter of the second post of greater than or equal to 25:1.
[0071] Example 10: The apparatus of one of examples 1 to 9, wherein
the substrate includes a silicon (Si) wafer having an aluminum
oxide (Al.sub.2O.sub.3) layer disposed thereon.
[0072] Example 11: A method comprising: providing a substrate;
forming a patterned catalyst layer on the substrate, the patterned
catalyst layer defining a template for carbon nanotube growth, the
template defining a pattern for formation of: a first carbon
nanotube post; a second carbon nanotube post; and a supporting
structure disposed between the first carbon nanotube post and the
second carbon nanotube post. The method further comprising growing
carbon nanotubes on the patterned catalyst layer to form the first
carbon nanotube post, the second carbon nanotube post and the
supporting structure, the first carbon nanotube post and the second
carbon nanotube post each having an aspect ratio of a height to a
diameter of greater than or equal to 25:1, the supporting structure
having an aspect ratio of a height to a width of greater than or
equal to 200:1; and removing the supporting structure, such that
each of the first carbon nanotube post and the second carbon
nanotube post are freestanding and extend substantially vertically
from the substrate.
[0073] Example 12: The method of example 11, wherein, prior to
removal of the supporting structure, a height of the first carbon
nanotube post, a height of the second carbon nanotube post and a
height of the supporting structure are substantially a same
height.
[0074] Example 13: The method of example 12, wherein the same
height is greater than or equal to 1 millimeter (mm).
[0075] Example 14: The method of one of examples 11 to 13, further
comprising infiltrating the first carbon nanotube post and the
second carbon nanotube post with carbon (C).
[0076] Example 15: The method of one of examples 11 to 14, further
comprising infiltrating the first carbon nanotube post and the
second carbon nanotube post with at least one of silicon (Si) and
silicon nitride (SiN).
[0077] Example 16: The method of one of examples 11 to 15, further
comprising plating the first carbon nanotube post and the second
carbon nanotube post with a metal.
[0078] Example 17: The method of one of examples 11 to 16, wherein
removing the supporting structure includes: performing a
non-directional plasma etch to remove an upper portion of the
support structure, such that a lower portion of the support
structure remains, the lower portion of the support structure being
disposed on the substrate; infiltrating the first carbon nanotube
post, the second carbon nanotube post and the lower portion of the
support structure with carbon (C); and performing a directional
plasma etch to remove the lower portion of the support
structure.
[0079] Example 18: The method of one of examples 11 to 18, wherein
the substrate includes a silicon (Si) wafer having an aluminum
oxide (Al.sub.2O.sub.3) layer disposed thereon, the patterned
catalyst layer being formed on the Al.sub.2O.sub.3 layer.
[0080] Example 19: The method of example 18, wherein forming the
patterned catalyst layer includes forming, using photolithography,
a patterned iron (Fe) layer, the Al.sub.2O.sub.3 layer preventing
diffusion of the patterned Fe layer into the Si wafer.
[0081] Example 20: An apparatus comprising: a substrate; and an
array of carbon nanotube posts disposed on the substrate, each
carbon nanotube post of the array of carbon nanotube posts
including a plurality of nanotubes and extending substantially
vertically from the substrate, each carbon nanotube post of the
array of carbon nanotube posts having an aspect ratio of a height
of the post to a diameter of the post of greater than or equal to
25:1.
[0082] It will also be understood that when an element, such as a
layer, a region, or a substrate, is referred to as being on,
connected to, electrically connected to, coupled to, or
electrically coupled to another element, it may be directly on,
connected or coupled to the other element, or one or more
intervening elements may be present. In contrast, when an element
is referred to as being directly on, directly connected to or
directly coupled to another element or layer, there are no
intervening elements or layers present. Although the terms directly
on, directly connected to, or directly coupled to may not be used
throughout the detailed description, elements that are shown as
being directly on, directly connected or directly coupled can be
referred to as such. The claims of the application may be amended
to recite exemplary relationships described in the specification or
shown in the figures.
[0083] As used in this specification, a singular form may, unless
definitely indicating a particular case in terms of the context,
include a plural form. Spatially relative terms (e.g., over, above,
upper, under, beneath, below, lower, and so forth) are intended to
encompass different orientations of the device in use or operation
in addition to the orientation depicted in the figures. In some
implementations, the relative terms above and below can,
respectively, include vertically above and vertically below. In
some implementations, the term adjacent can include laterally
adjacent to or horizontally adjacent to.
[0084] While certain features of the described implementations have
been illustrated as described herein, many modifications,
substitutions, changes and equivalents will now occur to those
skilled in the art. It is, therefore, to be understood that the
appended claims are intended to cover all such modifications and
changes as fall within the scope of the implementations. It should
be understood that they have been presented by way of example only,
not limitation, and various changes in form and details may be
made. Any portion of the apparatus and/or methods described herein
may be combined in any combination, except mutually exclusive
combinations. The implementations described herein can include
various combinations and/or sub-combinations of the functions,
components and/or features of the different implementations
described.
* * * * *