Note: Descriptions are shown in the official language in which they were submitted.
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MECHANICAL PROCESS FOR CREATING PARTICLES IN
A FLUID
CROSS-REFERENCE TO RELATED APPLICATION
100011 This application claims priority to U.S. Provisional Application No.
60/918,896 filed March 20, 2007, the entire contents of which are hereby
incorporated by
reference.
100021 The U.S. Government has a paid-up license in this invention and the
right
in limited circumstances to require the patent owner to license others on
reasonable terms
as provided for by the terms of NSF CAREER Grant No. CHE-0450022.
BACKGROUND
1. Field of Invention
100031 This application relates to processes and systems for making particles,
and
more particularly processes and systems for making particles having a
dimension less
than about 1 mm.
2. Discussion of Related Art
[0004] The contents of all references, including articles, published patent
applications and patents referred to anywhere in this specification are hereby
incorporated
by reference.
10005] An important emerging class of non-spherical colloidal materials are
microscopic and nanoscopic particles that have designed shapes and are created
by
lithographic means (see e.g. Hernandez, C.J.; Mason, T.G. Colloidal alphabet
soup:
Monodisperse dispersions of shape-designed LithoParticles. J. Phys. Chem. C
2007, 111,
4477-4480). (These will also be referred to as LithoParticles in this
specification.)
Optical pattern replicating systems, such as high-fidelity lens-based steppers
(Madou,
M.J. Fundaiueutals of Inicrofabricatioir: The scietace of miuiaturization. 2nd
ed.; CRC
Press: Boca Raton, 2002), typically used to print electronic structures on
coniputer chips,
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have been used to mass-produce LithoParticles and create Brownian dispersions
of an
entire particulate alphabet: "Colloidal Alphabet Soup"(Hernandez, C.J.; Mason,
T.G.
Colloidal alphabet soup: Monodisperse dispersions of shape-designed
LithoParticles. J.
Phys. Chem. C 2007, 111, 4477-4480). In the basic irr-plementation of this
approach, a
polymer resist layer can be cross-linked by the optical exposure and, after
development,
the polymer resist particles can be lifted off of the substrate. This optical
approach for
making LithoParticles has important and non-obvious differences from earlier
approaches
(Higurashi, E.; Ukita, H.; Tanaka, H.; Ohguchi, O. Optically induced rotation
of
anisotropic micro-objects fabricated by surface micromachining. Appl. Phys.
Lett. 1994,
64, 2209-22 10; Brown, A.B.D.; Smith, C.G.; Rennie, A.R. Fabricating colloidal
particles
with photolithography and their interactions at an air-water interface. Phys.
Rev. E 2000,
62, 951-960; Sullivan, M.; Zhao, K.; Harrison, C.; Austin, R.H.; Megens, M.;
Hollingsworth, A.; Russel, W.B.; Cheng, Z.; Mason, T.G.; Chaikin, P.M. Control
of
colloids with gravity, temperature gradients, and electric fields. J. Phys.
Condens. Matter
2003, 15, S 11-S 18) that required etching as part of the procedure. Although
robotically
automated optical exposure can be used to create significant quantities of
monodisperse
LithoParticles, expensive lithography exposure systems must be continuously
used to
optically pattern films during the particle production process. Due to the
limited
availability and expense of these precise optical exposure systems, there
would be
advantages to other LithoParticle production methods that could rapidly
produce shape-
designed particles without relying on such optical equipment during the
repetitive
production process.
100061 Mechanical imprinting, whether thern-ial or step-and-flash, is a
technology
that involves bringing two solid plates into contact after depositing a
desired material
between them (Madou, M.J. Fzuidainentals of inicrofcibricatiar: The scieitce
of
rniniaturizatioir. 2nd ed.; CRC Press: Boca Raton, 2002; Chou, S.Y.
Nanoimprint
lithography and lithographically induced self assembly. MRS Bulletin 2001, 26,
512;
Chou, S.Y.; Krauss, P.R.; Renstrom, P.J. Nanoimprint lithography. J. Vacuum
Sci. Tech.
B 1996, 14 (6), 4129-4133; Resnick, D.J.; Mancini, D.; Dauksher, W.J.;
Nordquist, K.;
Bailey, T.C.; Johnson, S.; Sreenivasan, S.V.; Ekerdt, J.G.; Willson, C.G.
Improved step
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and flash imprint lithography templates for nanofabrication. Microelectronic
Engineering
2003, 69, 412-419). Once the surfaces of the two plates touch, the niaterial
only fills
trenches or wells in one plate that has been prepared with the desired
patterns. Imprinting
essentially forces a desired material into voids that have been created in one
of the
surfaces to form a mold. While the two plates are touching (or nearly
touching), a
process, such as cross-linking in the case of polymers, can be used to
rigidify the material
in the mold, and then the plates are separated. During the separation, if the
release of the
desired material from the corrugated surface can be made efficiently, then the
result is a
set of raised structures of the desired material on the flat surface of the
other plate.
lniprinting is a subset of the more general process of embossing, in which a
mold is
pressed into the surface of a material that is not as rigid and then removed
to create raised
comigations that reflect the mold. However, by contrast to embossing,
mechanical
imprinting involves squeezing out material between two solid plates where they
touch, so
that only the negative relief corrugations in one plate become filled with the
desired
material.
100071 Performing mechanical imprinting reproducibly in a production setting
can
be problematic for many reasons. It is often difficult to achieve good
nlechanical contact
between the two plates over large surface areas. To mitigate this, large
sections of the
plates are often cut away so that only small, disconnected pedestals
containing the desired
patterns touch the flat plate. Using pedestals decreases the surface area and
production
rate significantly. Defects in the surfaces of the plates, dust, or enhanced
surface
roughness due to wear can preclude the exact contact of the plates, especially
for larger
substrate sizes. For very small shapes, the wetting properties of the material
to be
imprinted with the plates can play an important role in deterniining the
success and
reproducibility of the imprinting procedure. These are some of the primary
reasons why
mechanical imprinting has not been widely adopted by the electronics industry
as a
replacement to more reliable optical approaches. Although imprinting is making
some
inroads into certain specialty electronics applications, it is uncertain if
mechanical
imprinting technology will advance to a degree of robustness necessary to
overtake
existing optical methods in the current race to the sub-50 nm level. Although
it is
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possible to create LithoParticles using imprinting methods, as we and others
(Rolland,
J.P.; Maynor, B.W.; Euliss, L.E.; Exner, A.E.; Denison, G.M.; DeSimone, J.M.
Direct
fabrication of monodisperse shape-specific nanobiomaterials through imprinting
exists (J.
Am. Chem. Soc. 2005, 127, 10096-10100) , yet developing alternative approaches
for
rapidly mass-producing LithoParticles that do not involve imprinting or
repetitious
exposure by an optical lithography system would be highly useful.
SUMMARY
100081 A method of producing at least one of microscopic and submicroscopic
particles according to some embodiments of the current invention includes
providing a
template comprising a plurality of discrete surface portions, each discrete
surface portion
having a surface geometry selected to impart a desired geometrical property to
a particle
while being produced; depositing a constituent material of the at least one of
microscopic
and submicroscopic particles being produced onto the plurality of discrete
surface
portions of the template to form at least portions of the particles;
separating the at least
one of microscopic and submicroscopic particles comprising the constituent
material
from the template into a fluid material, the particles being separate from
each other at
respective discrete surface portions of the template; and processing the
template for
subsequent use in producing additional at least one of microscopic and
submicroscopic
particles. The method of producing at least one of microscopic and
submicroscopic
particles according to an embodiment of the current invention is free of
bringing a solid
structure, other than the constituent material, into contact with the template
proximate the
plurality of discrete surface portions during the producing, and is free of
bringing the
solid structure into contact with the constituent material during the
producing.
100091 A multi-component composition according to some embodiments of the
current invention includes a first material component in which particles can
be dispersed,
and a plurality of particles dispersed in the first material component. The
plurality of
particles is produced by methods according to embodiments of the current
invention.
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1000101 A system for manufacturing at least one of niicroscopic and
submicroscopic particles according to sonie enibodiments of the current
invention
includes a template cleaning and preparation system; a deposition system
arranged
proximate the template cleaning and preparation system to be able to receive a
template
fron-- the template cleaning and preparation system upon which material will
be deposited
to produce the particles; and a particle removal system arranged proximate the
deposition
system to be able to receive a template from the deposition system after
material has been
deposited on the template. The system for nianufacturing particles is free of
a structural
component, other than the constituent material, for contacting with the
template
proximate a plurality of discrete surface portions of the template, and is
free of a
structural component, other than the constituent material, for contacting with
the
constituent material during the producing.
BRIEF DESCRIPTION OF THE DRAWINGS
1000111 The invention is better understood by reading the following detailed
description with reference to the accompanying figures in which:
1000121 Figures 1 is a schematic illustration of a repeatable process for
making
LithoParticles using permanent Well-Deposition Particle Templating (W-DePT)
according to an embodiment of the current invention. Starting with the well-
template
(top), a sacrificial release layer is deposited, then the target particle
material is deposited,
and the particles in the bottoms of the wells are released by immersion and
agitation in a
fluid, which causes the sacrificial layer to dissolve (bottom). The
LithoParticles are
retained in the fluid, and the well-template is cleaned and re-used.
Optionally, the
pattemed film containing holes in the shapes of the particles can be retained
for use
and/or recycling.
1000131 Figure 2(a) is a schematic illustration of a method of producing a
well-
teniplate suitable for W-DePT according to an enibodiment of the current
invention. An
Si02 layer is deposited on a flat solid Si substrate and is then spin-coated
with a
photoresist layer. This top resist layer is exposed using an optical
lithography system.
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The exposed resist is developed, yielding a contirmous resist pattern that
contains holes
that reflect the desired particle shapes. Reactive ion etching of the exposed
Si02 regions
then exposes similarly shaped regions of the Si surface. Subsequent chlorine
etching to
the desired depth creates impressions of the desired well patterns in the Si
substrate, and
the residual photoresist and Si02 are stripped and removed.
1000141 Figure 2(b) sliows an SEM image of wells that have the desired square-
cross shape that have been etched into a silicon wafer using the method of
Figure 2(a).
1000151 Figures 3(a) -3(c) show optical micrographs of several stages of the
process described in Figure 1. Figure 3(a) shows a reflection micrograph of
the etched Si
well-template showing a high density of wells shaped in the form of square
crosses.
Figure 3(b) shows a reflection micrograph after depositing a 100 nni
sacrificial release
layer of water-soluble Omnicoat and after sputtering 70 nm gold onto the
release-treated
well-template. Figure 3(c) shows a transmission micrograph of gold particles
after fluid
assisted release out of the wells into an aqueous solution.
1000161 Figures 4(a)-4(b) show number-weighted size distributions of square
crosses produced by W-DePT, as measured using SEM images of fifty particles.
Figure
4(a) shows the distribution of the arm width, N(w), measured at the center of
an arm,
yields an average arm width <w> = 1.37 0.04 pm. Figure 4(b) shows the
distribution of
the total cross length, N(l), measured from the center of the end of one arm
to the center
of the end of the opposite arm, yields an average length <l> = 4.35 0.06 pm.
100017] Figure 5 is a schematic illustration of an example of a continuous
automated track production system for making LithoParticles using W-DePT
according to
an embodiment of the current invention. A sacrificial layer is deposited onto
a clean
well-template, the particle material is deposited, the well-template is
brought in contact
with a fluid and agitated to release the desired LithoParticles into the
fluid, and the well-
template is cleaned and dried, ready for the next cycle. Optionally, a
continuous
patterned film can be collected, separated, and potentially deposited on a
flat substrate to
produce an optical mask. All devices can be simultaneously operating using
multiple
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well-templates, and a robotic system can transfer the treated well-templates
between
devices.
1000181 Figure 6 is a schematic illustration of Well-Deposition Particle
Templating
using a permanent release layer that coats the well-template's surface
according to an
embodiment of the current invention. The desired particle material is
unifonnly
deposited onto the well-template in a direction perpendicular to the surface
of the
template. The deposited material does not adhere to the permanent release
layer, so
simple fluid agitation releases the particles without disturbing the release
layer. The
particles are separated and retained. Optionally, a film replica containing
holes of the
desired particle shapes can also be recovered. The well-template is then re-
used, and the
process is repeated.
[000191 Figure 7 is a schematic illustration of Well-Deposition Particle
Templating
through solidification of deposited materials according to an embodiment of
the current
invention. In this example, a permanent release coating has been initially
applied to the
well-template. Through deposition, the wells are filled with a material that
can be
solidified; a continuous surface layer may exist. This surface layer is
removed by spin-
coating or mechanical displacement. The particle material is solidified, and
the particles
are removed, separated, and retained through fluid-assisted lift-off. The well-
template is
then re-used and the process is repeated.
1000201 Figure 8 is a schematic illustration of Well-Deposition Particle
Templating
according to an embodiment of the current invention using a solid well-
template with
overhanging side-walls. The process is essentially the same as that described
for Figure
1; directional deposition of the particle material normal to the teinplate's
surface creates
islands of the desired particle shapes inside the wells. These islands do not
touch the
side-walls, so particle release is very efficient. It is not necessary to coat
the side-walls of
the wells under the overhang for this process to be successful. However, the
bottoms of
the wells must be coated with the release material.
1000211 Figure 9 is a schematic illustration to show that Well-Deposition
Particle
Templating according to an embodiment of the current invention may not work
properly
when a continuous layer of the particle material is formed over all of the
corrugated
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surfaces. In this example, the well-teniplate has been etched to create wells
that have
underhanging side-walls. Because these side-walls can accumulate the deposited
particle
material, even if directionally deposited normal to the template, separated
regions of
deposited particle niaterial cannot be formed, and no discrete particles can
be created or
released without removing the top continuous film by a process such as
abrasion or
polishing.
[00022] Figure 10 is a schematic illustration of Well-Deposition Particle
Templating according to an embodiment of the current invention to create non-
slab-
shaped pyraniid shell particles using a template that has wells coated with a
permanent
release agent. This method resembles that of Figure 1, but the bottom of the
well-
template has been patterned to provide a surface that is not completely flat.
The well-
template can be re-used and this process can be repeated.
1000231 Figure 11 is a schematic illustration of Well-Deposition Particle
Templating according to an embodiment of the current invention to create non-
slab-
shaped solid pyramid particles using a template that has wells with
underhanging side-
walls. This method resembles that of Figure 1, but there is an additional step
of removing
the continuous layer of particle material on the top contiguous surface of the
well-
template prior to fluid-assisted removal of the discrete particle shapes. As a
result, no
continuous film is created in this process. The well-template can be re-used
and this
process can be repeated.
1000241 Figure 12 is a schematic illustration of a repeatable process for
making
LithoParticles using permanent Pillar-Deposition Particle Teniplating (P-DePT)
according to an embodiment of the current invention. Starting with the pillar
template
(top), a sacrificial release layer is deposited, then the target material for
the particle is
deposited, and the particles at the tops of the pillars are released by
immersion into a fluid
and dissolution of the sacrificial layer (bottom). The LithoParticles are
retained in the
fluid (arrows at left bottom), and the pillar template is cleaned and re-used
(arrows at
right).
1000251 Figure 13(a) is a schematic illustration of a method of producing a
pillar
template suitable for P-DePT according to an embodiment of the current
invention. A
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flat solid substrate is coated with a resist layer; this resist layer is
exposed using a
lithography system, the exposed resist is developed and descunlmed, yielding
resist
islands that reflect the desired particle shape; the exposed substrate is
etched, the residual
photoresist is stripped away, and the etched substrate is cleaned.
[00026] Figure 13(b) shows a scanning electron micrograph of a pillar-template
for
making a plurality of plate-like particles that resemble square crosses. This
template is
made by ion etching a silicon surface according to an embodiment of the
current
invention.
[000271 Figuresl4(a)-14(d) show reflection optical micrographs for examples
according to an embodiment of the current invention. Figure 14(a) shows a 45
nm thick
gold layer that has been deposited on the tops of the silicon square cross
pillar-template
by sputtering. Below the gold layer is a 20 nm coating of a sacrificial
polymeric release
agent, OMNICOAT. Figure 14(b) shows fluid-assisted release of particles: the
pillar-
template is immersed in water and agitated to increase the rate of dissolution
of the
release layer. Figure 14(c) shows the pillar-template can then be re-used.
Figure 14(d)
shows liberated cross-shaped gold particles are separated and recovered in
aqueous
solution (optical transmission micrograph).
[00028] Figure 15 is a schematic illustration of an example of a continuous
automated track production system for making LithoParticles using P-DePT
according to
an embodiment of the current invention. Clean pillar templates are introduced
(top), and
adhesion promoter is applied, the sacrificial layer is deposited, the particle
layer is
deposited, the pillar template is brought in contact with a fluid and agitated
to release the
desired LithoParticles into the fluid, and the wafer is cleaned and dried
(bottom), ready
for the next cycle. All devices can be simultaneously operating using multiple
templates,
and a robotic system transfers the pillar templates between devices (arrows).
1000291 Figure 16 is a schematic illustration of P-DePT of complex non-slab
particle shapes using a permanent release coating according to an embodiment
of the
current invention. The tops of the pillars, which were originally flat, have
been etched to
provide a structured surface and then permanently coated with a release agent
(top of
Figure 16). As shown here, the top surface of the pillars may even be etched
to provide
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negative relief patterns, such as pyramidal or conical depressions. A desired
particle
material (bottoni of Figure 16) is deposited onto the surface, and fluid
agitation releases
the particles froni the pillars. The particles are retained in the fluid and
the pillar template
can be re-used.
1000301 Figure 17 is a schematic illustration of P-DePT of complex non-planar
particle shapes having uniform thickness using a permanent release coating.
The tops of
the pillars, which were originally flat, have been etched to provide a
structured surface
(e.g. a pointed pyramid or cone) and permanently coated with a release agent
(top of
Figure 17). The desired particle material (bottom of Figure 17) is deposited
onto the
release-coated sculpted pillar surfaces using a directional process that
creates a layer
having uniform thickness, and fluid agitation releases the non-planar
pyramidal particles
from the pillars. These non-planar LithoParticles are retained in the fluid,
and the pillar
template is re-used.
[000311 Figure 18 is a schematic illustration of another embodiment of P-DePT
according to the current invention.
[000321 Figure 19 is a schematic illustration of another embodiment of P-DePT
according to the current invention.
1000331 Figure 20 is a schematic illustration of another embodiment of P-DePT
according to the current invention.
1000341 Figure 21 is a schematic illustration of another embodiment of P-DePT
according to the current invention.
1000351 Figure 22 is a schematic illustration of another embodiment of P-DePT
according to the current invention.
[00036] Figure 23 is a schematic illustration of another embodiment of P-DePT
according to the current invention.
1000371 Figure 24 is a schematic illustration of another embodiment of P-DePT
according to the current invention.
[000381 Figure 25 is a schematic illustration of an example of a complex
relief
pattern according to an embodiment of the current invention. When reproduced
over the
entire surface of a template, such a pattern can be used to produce plate-like
particles in
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the shape of square slabs with up to 100% area coverage and efficiency. The
cutaway
view shown here is just a portion of the teniplate surface that shows how the
different
square levels can be configured with neighboring levels so that isolated
square particles
are produced over the entire surface of the teniplate. This template has been
constructed
to provide multiple levels of relief, not just simple pillars or wells. In
this exaniple, there
are six different relief levels, and directional deposition of the desired
particle material
from above (froni top of the page toward the bottom) onto the square-shaped
surfaces will
produce square-shaped particles froni all six different relief levels. A
single release step
could be used to release particles from all six levels into solution. This
would be an
efficient way of liberating particles from all of the surfaces at different
relief levels.
Alternatively, nniltiple release steps could be used to release particles from
each of the
six different levels of the template.
DETAILED DESCRIPTION
1000391 In describing embodinients of the present invention illustrated in the
drawings, specific terminology is enlployed for the sake of clarity. However,
the
invention is not intended to be limited to the specific terminology so
selected. It is to be
understood that each specific element includes all technical equivalents which
operate in
a similar manner to accomplish a similar purpose.
100040] Some embodiments of the current invention provide methods for
producing microscopic and/or submicroscopic particles. The methods according
to some
embodiments of the current invention include providing a template that has a
plurality of
discrete surface portions, each discrete surface portion having a surface
geometry selected
to impart a desired geometrical property to a particle while being produced.
Each of the
discrete surface portions can be, but are not limited to, a flat surface, a
curved surface, a
complex contoured surface, a surface with a plurality of subsurface regions,
or any
combination thereof. Herein, microscopic refers to the range of length scales
equal to and
greater than one micrometer, including length scales ranging up to about one
millimeter.
Herein, submicroscopic refers to the range of length scales below one
micrometer,
including length scales ranging down to about one nanometer.
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[000411 The methods according to some embodiments of the current invention
also
include depositing a constituent niaterial of said at least one of microscopic
and
submicroscopic particles being produced onto said plurality of discrete
surface portions
of said template to form at least portions of said particles. The constituent
material is a
rriaterial in the composition of the particles being manufactured. The broad
concepts of
the current invention are not limited to any specific constituent materials.
There is an
extremely broad range of materials including organic, inorganic, composite,
multi-
component and any combination thereof that could be used in various
embodiments of
the current invention. The depositing can be a directional deposition in some
embodiments of the current invention that, for example, leaves at least a
fraction of wall
portions around the discrete surface portions uncoated by the constituent
material. The
depositing can include spin-coating, spray-coating, dip-coating, sputtering,
chemical
vapor deposition, molecular beam epitaxy, electron-beam metal deposition, or
any
combination thereof in some embodiments of the current invention.
1000421 The methods according to some embodiments of the current invention
further include separating at least one particle from the template in which
the particle
separated has the constituent material in its composition. The particle may be
separated
into a fluid, for example, into a liquid in some embodiments of the current
invention. In
some embodiments there may be one or a small number of particles separated
from the
template, but in other embodiments, there can be a very large number of
particles
separated in the same separation step. For example, in some embodiments there
could be
hundreds of thousands, millions and even billions or more particles separated
from the
template in the same step.
[00043] The methods according to some embodiments of the current invention
further include processing the template for subsequent use in producing
additional
particles. Once the template is processed for subsequent use, the above-noted
depositing
and separating steps can be repeated to produce additional particles. The
template may be
reprocessed many times according to some embodiments of the invention to mass
produce, in assembly-line fashion, very large numbers of the particles. The
method of
producing particles according to such embodiments of the current invention
does not
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include pressing a structural component against the template to control the
application of
material to the template, such as is done with printing methods.
Well-Deposition Particle Templating
1000441 An embodiment of the current invention is a process which will be
referred to as "Well-Deposition Particle Templating" (W-DePT). W-DePT involves
only
a single patterned solid plate and an appropriate deposition and release
scheme. A solid
"well-template" is created by permanently etching a solid surface to make one
or more
wells that reflect the desired shape or shapes. Although optical or electron
beam (e-
beam) lithography is typically used in combination with etching to first make
this "wel l-
template", the reinaining steps that are repeated for mass-producing particles
do not
require any exposure or etching systems.
[000451 In a simple implementation, W-DePT can be achieved by: (1) depositing
a
thin layer of a release agent, such as a temporary sacrificial release layer
(e.g. fluid-
soluble polymer) or a permanent molecular coating (e.g. fluorinated siloxane
chains) over
the corrugated surface of the well-template; (2) depositing the desired
particle materials at
a desired thickness through various deposition processes, such as sputtering,
physical
vapor deposition (PVD), chemical vapor deposition (CVD), or spin-coating; and
then (3)
releasing the particles from the wells into a fluid, usually using some form
of agitation
(See Figure 1). Fluid-assisted release can involve dissolving a temporary
sacrificial
release layer, or it can simply dislodge particles from a surface that may be
coated with a
permanent release agent. Since the well-template is not altered by the
deposition and
release processes, it can be re-used, and the templating process can be
rapidly repeated.
We have used W-DePT to mass-produce particles having less than 5%
polydispersity in
thickness and linear cross-sectional dimensions with an efficient release rate
exceeding
99%. By performing W-DePT using multiple templates simultaneously,
LithoParticle
production rates can be made very high without the difficulties and added
complexities of
imprinting methods involving mechanical contact of a flat plate with a
patterned surface.
Moreover, repeated patterned optical exposure is also not necessary to achieve
a high-
throughput production scheme.
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W-DePT Exaniples
Methods for Producing a Well-Template
1000461 Many lithographic methods can be used to create a patterned "well-
template" suitable for W-DePT. As an example, we describe one approach that
can be
used to create a well-template for making cross-shaped particles with W-DePT.
This
process is shown schematically in Figure 2(a). A densely populated optical
reticle-mask
(not shown) of chrome on quartz that contains patterns of many disconnected
cross-
shapes is designed and produced using e-beam lithography following standard
methods
(Madou, M.J. Faurclanzentals ofmicrofcabriccitioir:
Thescierzceofnziiiiatitrizatioii. 2nd ed.;
CRC Press: Boca Raton, 2002). This optical reticle-mask is not required for
producing a
well-template, but it provides a convenient means of more easily producing
more than
one well-template from an optical, rather than an e-beam, process. If only one
template is
desired or if the desired resolution lies below the optical limit, an e-beam
exposure
system could be used to directly pattern a resist layer, and subsequent
etching could
provide the well-template without any need for an optical reticle-mask.
Assuming that
the optical reticle-niask has been produced, a flat polished silicon wafer is
coated with
170 nm of silicon dioxide using plasma-enhanced CVD and then a 1.6 micron
layer of
polymer photoresist (Shipley AZ-5214) using a spin-coater at 3,000 RPM. A
mercury i-
line projection stepper system (Ultratech XLS-2145i), exposes the resist-
coated wafer
with patterned ultraviolet light that has passed through the reticle-mask.
After normal
development, the crosslinked resist forms an interconnected layer that
contains many
voids in the form of square crosses. Inside these voids, the silicon dioxide
layer is
exposed. A reactive ion etcher (RIE) is used to completely etch through the
oxide layer,
revealing the silicon surface. This exposed silicon surface is permanently
etched using a
chlorine etcher to a depth of 0.8 microns, creating many wells in the shapes
of crosses.
The residual protective resist and remaining oxide are then stripped (i.e.
removed) from
the silicon surface using piranha (a mixture of 70% sulfuric acid and 30%
hydrogen
peroxide) and an aqueous solution of HF (50%). Depending upon the desired
particle
size and shape, the resulting well-template on a five-inch silicon wafer can
contain up to
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one billion or more wells (i.e. negative relief features) that define the
desired particle
shapes in negative relief, shown in the scanning electron micrograph of Figure
2(b). The
area fraction of the wells defining the desired shapes can be low, although
there is an
advantage to having a higher density for particle production throughput,
provided the
wells remain discrete and do not interconnect.
[000471 Choosing appropriate etching conditions and rates is important in some
embodiments in order to obtain uniform side-walls without undesirable defects,
such as
pronounced scalloping, that could inhibit release. Furthermore, the etch depth
has been
made larger than the niaximum desired thickness of the particles. Extremely
high etch
depths of many microns may not be desirable in some embodiments since deeper
wells
can reduce the rate and efficiency of release of particles that are formed in
them. The
basic requirement for the template according to this embodiment of the
invention is that it
is a solid material containing a permanent patterned structure of wells that
define desired
particle shapes. Usually, polished solid materials, such as silicon or quartz
wafers,
represent the easiest candidates for patterning at length scales less than ten
microns for
making colloidal particles. However, materials other than silicon and quartz
can be used
for the well-template.
1000481 A wide variety of lithographic approaches other than the one we have
described can be used to produce the patterned "well-template". These
approaches may
not involve depositing a silicon oxide layer onto a silicon wafer, performing
resist-based
optical lithography to print the repeating disconnected patterns of particle
shapes, nor
etching silicon dioxide, as we have described in our example. The key
characteristic of a
well-template according to this embodiment of the invention is essentially a
solid
material that has at least one surface that has been permanently patterned to
have one or
more wells of a desired shape into which at least the desired particle
material can be
deposited.
Mass-Producing Particles Using Well-Deposition Templating
1000491 Once the well-template has been made, LithoParticles can be mass-
produced by a succession of steps that involve deposition and fluid-assisted
release. As
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an example, using the well-template of square-crosses, we produce an aqueous
suspension of cross-shaped gold particles by the process outlined in Figure 1.
We coat all
of the surfaces, including the side-walls, of the well-template with a release
agent. This
could be a simple permanent molecular layer, such as a fluorocarbon, that
provides low
surface energy contact with the desired material for the particles, or it
could be a layer of
deposited sacrificial material (e.g. a water-soluble polymer) that can be
removed in a
subsequent release step. For our example here, we use the second alternative.
When
necessary, we treat the well-template with an adhesion promoter,
hexamethyldisilazane
(HMDS), in order to facilitate the process of uniformly coating of the
sacrificial material
over all surfaces of the patterned well-template, including the side-walls.
For exaniple,
we create a thin water-soluble sacrificial release layer (e.g. Omnicoat) over
the surface of
the well-template by spray-coating using an atomizer (e.g. air-brush),
spinning the wafer
at 3,000 RPM to remove any excess polymer solution that remains on the top
surface of
the wafer. Baking at 200 C for one minute evaporates the solvent for the
release agent,
leaving behind a thin solid layer that uniformly coats all surfaces of the
well-template to a
thickness of approximately 100 nm. Next, we uniformly deposit the desired
thickness, 70
nm, of the desired particle material, gold, onto the coated well-template
using sputtering.
Although this deposition also coats the top surface of the template, notjust
the wells, the
top surface layer is completely interconnected over macroscopic length scales,
so there
are no small particles that would be formed from this top layer of deposited
material.
After depositing the desired particle material, the well-template is immersed
in Omnicoat
developer (2.28% tetramethyl ammonium hydroxide), and agitated in the
developer using
an ultrasonic bath to cause the sacrificial layer to rapidly dissolve and the
gold particles to
be released into solution, as shown in Figure 3(c). The time necessary to
release the
particles from the wells is typically about two minutes. Care must be taken
not to make
the ultrasonic agitation too severe; otherwise, particles can be broken by the
agitation.
1000501 As a by-product of the W-DePT process, a large interconnected film of
the
desired particle material is created. In the exaniple given above, a layer of
patterned gold
with cross-shaped holes is also created and lifted off into solution at the
same time as the
particles. In principle, the intact patterned film could be used to create an
optical mask by
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deposition onto a quartz surface or for shape-specific filtration if mounted
on an
appropriate porous substrate. Because this filni is much larger than the
particles that are
produced, it can be easily separated from the particles during or after the
fluid-assisted
release process. If the particle material is valuable and a continuous film is
not a desired
product, then this interconnected layer can be recovered and potentially
recycled. In
practice, thin continuous films can be very fragile, and more vigorous
agitation used to
release particles can potentially tear or break them into smaller pieces. As a
result, mild
agitation that does not lead to release of the particles can be used to
recover an intact film
after lift-off, and subsequent stronger agitation can be used to release the
particles.
1000511 Scanning electron microscopy (SEM) images reveal that the number-
weighted polydispersity of the arm lengths and thicknesses of the crosses to
less than 5%.
In Figures 4(a) and 4(b), we show the size distributions N(w) and N(Z)
corresponding to
the width, w, of the arms of the crosses (measured at the middle of the arm)
and the total
end-to-end length, 1, of the arms of the crosses, respectively. We find that
the number-
weighted average width is <w> = 1.37 0.04 um and the average total length is
</> =
4.35 0.06 um, where uncertainties correspond to the standard deviations of
the
respective distributions. The polydispersity of the thickness, t, is more
difficult to
measure for thin particles that tend to deposit flat onto the conducting
surface, and we
estimate the average thickness to be approximately <t> ~z 70 nm. Based on
uniformity of
coatings sputtered on flat surfaces, we estimate the uncertainty in the
thickness of the
ensemble to be about 5 nm - 10 nm. More precise deposition devices that spin
and rotate
the substrate while they deposit, such as those used to create thin coatings
on optical
lithography masks, can provide a higher degree of uniformity in thickness over
a larger
surface area.
1000521 The polydispersity of the edge lengths is essentially set by the
precision of
the well-template (i.e. through the exposure and etching processes), whereas
the
polydispersity of the thickness by the uniformity of the deposition process
for coating the
wells with the desired materials. For the example W-DePT implementation that
we have
described using a polymer release layer and gold, the surface roughness of the
top and
bottom flat layers of the particles is determined by the roughness of the
deposited
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polymer layer and the uniformity of the sputtering process. We have performed
W-DePT
using the same template repeatedly without any noticeable degradation of the
well-
template or deterioration of the particle uniformity. Occasionally, the
surfaces of the
silicon well-template can be non-destructively cleaned using piranha and HF
solutions to
ensure maximum fidelity. For the method we have described to make gold
crosses, using
optical reflection microscopy, we estimate the efficiency of release to be
greater than 99%
after agitating for less than two minutes using an ultrasonic bath, with less
than one
particle in a thousand remaining stuck in a well. Non-directional vapor
deposition of the
sacrificial layer, rather than spray-coating and spin-coating, would most
likely improve
this release efficiency. It is obvious that this approach for making particles
will not be
successful if the sides of the wells become coated with the particle material,
thereby
connecting the continuous film on top of the template to the particles in the
wells. So,
directional deposition methods that do not coat the side-walls, such as
sputtering
deposition or evaporative deposition normal to the surface or using physical
vapor
deposition (e.g. thermal or e-beam), offer distinct advantages for the simple
example of
W-DePT that we have shown. Likewise, W-DePT may not yield discrete particles
in its
simplest form if the particle layer becomes too thick due to over-deposition,
such that the
material in the wells would form rigid contacts with the top continuous film.
[00053] Completely automated W-DePT can be performed in parallel using many
templates that are continuously recirculated by a robotic track system.
Identical well-
templates are circulated into a spray/spin coater, a baker, a sputterer, a
fluid agitation
bath, a cleaning tank, a drying stage, and then back to the spray/spin coater
to complete
the loop (see Figure 5). The spray/spin coater, baker, cleaning tank and/or
drying system
can be components of a template cleaning and preparation system of a system
for
manufacturing particles according to an embodiment of the current invention.
The
sputterer is one example of a possible deposition system for manufacturing
particles
according to an embodiment of the current invention. The deposition system is
not
limited to only a sputterer and may include other deposition systems including
those
described in references to various examples herein. The ultrasonic bath is one
example of
a possible particle removal system according to an embodiment of the current
invention.
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However, systems for manufacturing particles according to various embodiments
of the
current invention are not liniited to this specific exaniple. At the fluid
agitation stage,
LithoParticles are collected and retained in a fluid. The track systeni is
only one possible
way of performing high-throughput production. A rotary carrousel that provides
parallel
processing of several identical well-templates could also be used.
Alternatively, various
operations could be performed on specific regions of a well-template as it is
rotated or
translated, if these deposition methods can be scaled down. One of the main
advantages
of the automated parallel W-DePT replication process is that it doesn't
require a full-time
robotic optical exposure system; this system usually represents the most
expensive part of
any lithographic fabrication production line.
Well-Deposition Particle Templating: Permanent Release Layer
[00054] A simple alternative method for making the LithoParticles using W-DePT
involves permanently bonding a low-surface energy release agent to the
surfaces of the
well-template. This release agent can take the form of a fluorocarbon,
fluorohydrocarbon, or fluoro-siloxane with appropriate reactive groups for
bonding these
molecules to the well-template surfaces. This type of low-surface energy
coating can be
applied using standard methods of surface treatment. After treating the well-
template by
coating and bonding a high surface density of such molecules to all of the
patterned
surfaces, the treated well-template surface will have only a very weak
attractive
interaction with a desired particle material. Once this particle material has
been
deposited into the wells, the permanent release coating permits facile fluid-
assisted
release of particles from the wells without the need for the fluid to dissolve
a sacrificial
release layer. In this variation of W-DePT, shown in Figure 6, directional
deposition
normal to the template's surface yields particles in the wells and a
continuous tilm on the
top surface. Fluid-assisted release involving agitation can dislodge the
particles from the
wells and the upper continuous film without any deposition and removal of a
sacrificial
layer.
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Well-Deposition Particle Templating: Soliditication of a Material in the Wells
1000551 Another interesting variation of W-DePT, which can employ either a
teniporary or permanent release layer, involves depositing a desired target
particle
material in a liquid base into the wells and causing a solidification of that
target material
by some otller process, such as aggregation, gelation, phase changes due to
temperature or
pressure, or evaporation. This process is shown in Figure 7 for the case of an
inorganic
silicon dioxide (i.e. silica) xerogel (Himcinschi, C.; Friedrich, M.; Murray,
C.; Streiter, I.;
Schulz, S.E.; Gessner, T.; Zahn, D.R.T. Characterization of silica xerogel
films by
variable-angle spectroscopic ellipsometry and infrared spectroscopy. Semicond.
Sci.
Technol. 2001, 16, 806-811). Deposition of the sol liquid into the wells is
achieved by
spray coating, and then removing residual liquid from the well-template top
surface by
spin coating with the pure solvent. Heat treatment causes a porous gel of the
silicon
dioxide to form in each of the wells, and, by fiirther heat treatnient, these
gel particles can
be made to contract uniformly to form a more dense porous glass which retains
the
original shape of the wells. The shrunken particles retain the shapes of the
well, and the
efficiency of recovery for this method can be quite high. Due to the
contraction, this
method would work well for shapes such as square crosses, but may be
problematic for
shapes that contain holes. For such toroidal shapes, or donuts, it may be
necessary to
liberate the gel at an early stage from the wells before applying fiirther
heat treatment to
shrink the particles outside of the wells.
Well-Deposition Particle Templating: Overhanging Side-Walls
[00056] Well-templates that have overhanging side-walls (Madou, M.J.
Fuirclanaentals of microfabrication: The science of rnziiriaturizcttiori. 2nd
ed.; CRC Press:
Boca Raton, 2002) can be used for W-DePT, provided directional deposition of
the
desired target material for the particles is used. For instance, for gold
deposition normal
to the surface of an overhang well-template, particles can still escape from
the wells
during the release step, as shown in Figure 8. In this case where directional
deposition is
normal to the template surface, it is not necessary for the release material
to coat the side-
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walls of the wells in order for release to be feasible. It is sufficient for
the release
material to coat just the bottoms of the wells in the region where the
particle material is
deposited. Because the constriction at the top of the wells with overhanging
side-walls is
no larger than the deposited particle material, the particles can be released
from the wells.
Some forms of non-directional deposition into wells that have overhanging side-
walls
could create particles that are larger than the constriction. This situation
could preclude
W-DePT, because the constriction at the top of the wells could inhibit the
release of the
particles, even if they have been successfully liberated from the bottoms of
the wells.
Well-Deposition Particle Templating: Limitations- Underhanging Side-Walls
[00057] Several situations can lead to difficulties with the efficiency of
production
and release of particles by basic forms of W-DePT. The simplest W-DePT
approaches
may not produce well-separated and discrete particles if a well-template has
side-walls
that are "underhanging", rather than vertical or overhanging. For instance,
deposition of
the particle material into wells that have beveled underhanging side-walls,
created by
anisotropically etching silicon (Powell, 0.; Harrison, H.B. Anisotropic
etching of { 100}
and {l 10} planes in (100) silicon. J. Micromech. Microeng. 2001, 11, 217-
220), could simply create a continuous layer of the desired particle material
over a sacrificial release
layer, as shown in Figure 9. Regardless of the structure of the side-walls, if
the
deposition of particle material, whether by directional processes or not,
completely caps
off and separates a sacrificial release layer underneath the particle layer
from the fluid,
then fluid-assisted particle release will be precluded. This could also occur
if the release
material does not adequately coat the side-walls, causing the particle
material to touch
and potentially stick to the side-walls. Moreover, even if the particle
material does not
strongly adhere to the side-walls, particle material could also cut off access
of the fluid to
the release layer underneath the particle material in the wells. This would
prevent the
fluid from dissolving the sacrificial release layer, so release of the
particles could not
ensue. Even if the side-walls were coated with the sacrificial release agent,
if a very thick
layer of particle material is deposited into the wells, the process of
dissolving the release
material between the side-walls of the wells and the particle material filling
the wells
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would be slow. Flushing or flowing the fluid over the surface of the well-
template could
speed up the release.
1000581 Thus, one of the requirements of the simplest versions of W-DePT is
that
the deposition onto the well-template should create separate, disconnected
regions of the
desired particle material in each of the wells. The efficiency and rate of
release of the
LithoParticles from the wells can depend strongly on the thickness of the
sacrificial layer,
the side-wall geometry of the wells, and the method of deposition of both the
sacrificial
and particle layers. If the release layer is very thin on the side-walls, then
the convective
hydrodynamic penetration of the fluid to dissolve the release layer underneath
the
particles in the wells can be slow, because the region where it can penetrate
is more
highly constricted. Ultrasonic agitation can be used to expedite the release
process, but
even this more extreme form of agitation may fail. The combination of the well-
template
structure and the deposition steps should be chosen in such a manner as to (1)
provide
discrete structures of the desired particle material in the wells, (2) ensure
that these
discrete particle structures can be essentially completely liberated from the
wells on the
well-template, and (3) preserve the stn.ictural fidelity of the well-template
so that it can be
re-used.
Well-Deposition Particle Templating: Complex Three-Dimensional Shapes
[000591 The bottoms of the wells in the well-template need not be flat, and if
they
are appropriately shaped by either deposition or etching processes (Powell,
0.; Harrison,
H.B. Anisotropic etching of { 100} and { 110} planes in (100) silicon. J.
Micromech.
Microeng. 2001, 11, 217-220), it is possible to create particles that have
highly complex
three-dimensional geometries. In Figure 10, we show a variation of the basic
process in
which the bottom of the well-template has been etched to form a complex
contour, such
as a pyramid-shaped (or conical) well-bottom. 1n this example, the well-
template has
been treated with a permanent release agent, although a sacrificial release
agent could
also be used. Directional deposition of a layer having constant thickness
normal to the
template surface and fluid-assisted release lifts off shell-like
LithoParticles resembling
pyramids (or cones) that retain the contours of the well-bottom. Although
engineering
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the well-template will typically be more complex than for simple flat-bottomed
wells,
once the teinplate has been created, the particles can be mass-produced by
repeating only
the deposition and release processes.
Well-Deposition Particle Templating: Remove Deposited Material to Free
Particles
[000601 W-DePT can be used to make particles that are not slab-like, even with
undercut well-templates, if the top continuous layer of material can be
removed by a
process without also removing material deposited into the wells. This can be
achieved by
processes such as, for liquid-borne materials, by spinning off the top
continuous layer in a
whole surface process reminiscent of edge bead removal of resist at the edges
of wafers
(Madou, M.J. Fundamentals ofrnicrofabricatiorr: Tlaescience
ofnzirziaturization. 2nd ed.;
CRC Press: Boca Raton, 2002). For instance, it would be possible to make
particles such
as solid pyramids by etching a well-template that has indentations in the form
of
pyramids, depositing a release layer and then particle materials, spiinning
off the top
surface of the deposited particle layer (thereby creating disconnected islands
of particle
materials in the wells), solidifying the material in the wells, and releasing
the particles
from the wells, as we show in Figure 11.
[000611 Many variations of deposition of the sacrificial layer and for the
target
material layer are possible once the well-template has been made. These
materials
include organics (e.g. polymers), natural and synthetic biomolecules,
inorganics (e.g.
conductors, semi-conductors, insulators, including nitrides and oxides), metal-
organic
frameworks (MOFs)( Roswell, J.; Yaghi, O.M. Effects of functionalization,
catenation,
and variation of the metal oxide and organic linking units on the low-pressure
hydrogen
adsorption properties of metal-organic frameworks. J. Am. Chem. Soc. 2006,128,
1304-
1315), and nietals, or combinations of any of these compositions. Particles
can be
comprised of dense solids, porous solids, flexible solids, or even tenuous
gels.
LithoParticles made using W-DePT can also contain nanoscopic particulates,
such as
quantum dots, gold or silver nanoclusters, magnetically-responsive iron oxide,
or
molecules, such as fluorescent dyes or biologically active drugs. Performing
multiple
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depositions of different desirable target materials prior to the release step
can be used to
make hybrid bi-layer or multi-layer particles. These deposition methods
include, but are
not limited to, spin-coating, spray-coating, dip-coating, sputtering, chemical
vapor
deposition (CVD), molecular beam epitaxy (MBE), and electron-beam metal
deposition
(EBMD). Release can be made into aqueous or non-aqueous solvents for further
chemical surface treatment to increase particle stability against aggregation.
Particle
release could take place in a wide range of fluids, including supercritical
fluids or even
gases, not just liquids.
[00062] Well-Deposition Particle Templating is considerably different than
mechanical imprinting of features including discrete particle shapes. To
perform W-
DePT, no mechanical lithography device for imprinting, necessary to ensure
good
mechanical contact between two plates everywhere over the entire surface of
the wafer, is
needed. Moreover, the performance of W-DePT in reproducibly creating shapes
repeatedly from the same template is not nearly as sensitive to dust, wear,
and surface
imperfections as mechanical imprinting. Instead, to make LithoParticles, only
a single
patterned substrate, the "well-template", is required, along with an
appropriately chosen
deposition and release method. The internal feature sizes and overall
dimensions of the
particles are not limited to the microscale; direct e-beam writing, x-ray
lithography, or
deep-UV lithography to a resist-coated surface and subsequent etching could
make
templates with internal particle features, such as arm widths on the crosses,
and overall
particle lateral dimensions, smaller than 50 nm.
Pillar-Deposition Particle Templating
1000631 According to another embodiment of the current invention,
LithoParticles
can be mass-produced from a solid template that has been permanently etched to
make
pillars that define their cross-sectional shape in a process called "Pillar-
Deposition
Particle Templating" (P-DePT). Although making the patterned pillar-template,
which
may contain billions of replicas of a portion of a desired particle shape or
different shapes
in positive relief, can rely on optical or electron beam (e-beam) lithography,
the
remaining steps for particle production do not. A simple implementation of P-
DePT
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consists of the following steps: coating the pillars with a thin layer of a
release agent,
such as a sacrificial layer of water-soluble polynler; depositing the desired
particle
materials at a desired thickness through various deposition processes, such as
sputtering,
chemical vapor deposition (CVD), or spin-coating; and then releasing the
particles from
the pillars into water by dissolving the sacrificial layer using an aqueous
solution, as
shown in Figure 12. Since the pillar template can be re-used, the process can
be rapidly
repeated, and P-DePT is highly effective at producing particles with less than
5%
polydispersity in thickness and linear cross-sectional dimensions with a very
efficient
release rate exceeding 99%. By performing P-DePT using multiple pillar-
teniplates in
parallel, it is possible to increase production rates without having to also
increase the
nuniber of optical exposure systems.
[000641 The P-DePT method can facilitate the large-scale production of new
kinds
of soft multi-phase materials, particularly dispersions of particulates in
viscous liquids
(Russel, W.B.; Saville, D.A.; Schowalter, W.R. Colloiclal dispersioirs.
Canibridge Univ.
Press: Cambridge, 1989). These particles can be used as interesting probes for
applications such as microrheology (Mason, T.G.; Ganesan, K.; van Zanten,
J.H.; Wirtz,
D.; Kuo, S.C. Particle tracking microrheology of complex fluids. Phys. Rev.
Lett. 1997,
79, 3282-3285; Cheng, Z.; Mason, T.G. Rotational diffusion microrheology.
Phys. Rev.
Lett. 2003, 90, 018304) or bio-microrheology (Weihs, D.; Mason, T.G.; Teitell,
M.A.
Bio-microrheology: A frontier in microrheology. Biophys. J. 2006, 91, 4296-
4305).
Concentrated dispersions of solid shape-designed particles could exhibit
interesting
liquid-crystalline phases and exotic phase transitions as the particle volume
fraction is
increased quasi-statically. Moreover, by rapidly concentrating the particles
in the liquid,
one may quench in glassy disorder (Torquato, S.; Truskett, T.M.; Debenedetti,
P.G. Is
random close packing of spheres well defined? Phys. Rev. Lett. 2000, 84, 2064-
2067).
Understanding how the shape of the particles can influence jamming (Donev, A.;
Cisse,
I.; Sachs, D.; Variano, E.A.; Stillinger, F.H.; Connelly, R.; Torquato, S.;
Chaikin, P.M.
Improving the density of jammed disordered packings using ellipsoids. Science
2003,
303, 990-993) in concentrated dispersions can provide key insights into the
structure and
dynamics of disordered soft materials.
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P-DePT Examples
100065] To create a solid pillar-template suitable for P-DePT, as an example,
we
begin by creating a reticle mask containing a plurality of disconnected cross-
shapes
suitable for optical lithography; this reticle mask can be designed using
computer aided
design software and stored electronically in a digital file, and the mask can
be produced
from the digital file using a standard e-beam lithography writing system (e.g.
MEBES).
Using a mercury i-line stepper exposure system (Ultratech XLS-7500),
ultraviolet light
passes through the reticle's clear cross shapes to expose a one micron thick
resist-coated
(Shipley AZ-5214) flat silicon wafer. Following development and de-scumming,
which
removes the unexposed resist from the wafer's surface, a pattern of raised
crosses of
cross-linked resist remains on the wafer's surface, and the wafer is
permanently etched
using a reactive ion etcher to a depth of 8 microns in the regions outside the
crosses
where the wafer is exposed and unprotected. The residual protective resist is
then
stripped and the wafer is cleaned using piranha (a mixture of 70% sulfuric
acid and 30%
hydrogen peroxide). This process is shown schematically in Figure 13(a). The
resulting
pillar-deposition particle template on a five-inch silicon wafer contains
roughly one
billion raised pillars that define the desired particle shapes, shown in the
scanning
electron micrograph of Figure 13(b). Because the top surfaces of the pillars
have been
protected by the photoresist, they remain flat. The side surfaces of the
pillars may have
irregularities; as shown in this example, these will not affect the particle
production
process by the pillar method. Furthermore, the etch depth has been made larger
than the
desired thickness of the particles. Extremely high etch depths may not be
desirable since
pillars would become more susceptible to breakage from accidental mechanical
contact or
agitation of the template. Generally, an etch depthof at least twice the
maximum particle
thickness is appropriate. Other alternative approaches that yield the same
permanent
pillar template structure, such as depositing a silicon oxide layer onto a
silicon wafer,
performing resist-based lithography to print the repeating disconnected
patterns of
particle shapes, and etching the silicon dioxide, could also be used.
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1000661 Using the pillar-template, as an example, we produce an aqueous
suspension of cross-shaped gold plate-like particles according to general
scheme of
Figure 12. Since surfaces containing pillars, such as lotus leaves, are known
to produce
high effective contact angles for liquids that can make deposition ofliquid-
based polymer
solutions problematic, an adhesion promotor, HMDS, is applied to the silicon
by vapor
condensation. Next, a thin water-soluble sacrificial release layer (e.g.
Omnicoat) is then
spin-coated at three thousand RPM to provide a thickness of approximately 20
nm onto
the pillars and then baked at 200 C for 1 minute. The desired thickness of
gold, 45 nm,
is deposited uniformly onto the surface using sputtering. An optical
micrograph of the
top surface of the coated pillars is shown in Figure 14(a). Following the
deposition of the
desired particle material, the coated pillar-template is immersed in water,
and agitated to
cause the sacrificial layer to dissolve and the particles to be released into
solution, as
shown in Figures 14(b)-14(d). Typically, 2 minutes of agitation in an
ultrasonic bath is
adequate. Care must be taken so that the intensity of ultrasonic agitation is
not so severe
that it would cause released particles to break apart or damage the pillars on
the template.
100067] Using scanning electron microscopy, we have characterized the number-
weighted polydispersity of the arm lengths and widths of the crosses to be
about 2%. The
polydispersity of the thickness is more difficult to measure for such a thin
layer, and we
estimate it to be about 45 5 nm. The polydispersity of the edge lengths is
essentially set
by the precision of the pillar template (i.e. through the exposure and etching
processes),
and the polydispersity of the thickness by the uniformity of the deposition
process for
coating the pillars with the desired materials. In general, for directional
deposition of the
desired particle material, we do not observe overhangs, burs, or other
defects, and the
side-walls are flat. Other forms of deposition, such as solution delivery of a
desired
organic material, to the tops of the release-coated pillars and subsequent
baking could
lead to rounding of the top comers of the particles by liquid surface tension.
This may be
a desirable feature in some cases.
1000681 The P-DePT process can be repeated many times without degradation of
the pillar template. If deposited materials accumulate in the interconnected
trenches
beneath the pillars, occasionally, it may be necessary to clean off this
excess material by
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dipping the wafer in piranha or HF solutions. If the trenches are also coated
with a
release agent when the tops of the pillars are coated, then large continuous
interconnected
regions of the deposited material containing negative images of the desired
particles can
also be released into solution. These regions can be easily separated from the
particles
through sedimentation or filtration, since they are typically tens to hundreds
of microns in
size.
1000691 We have characterized the rate of efficiency of the lift-off of the
particles
from the pillars by using optical reflection microscopy to examine the tops of
the posts
after the sacrificial layer has been dissolved. When the sacrificial layer is
properly coated
over all of the tops of the pillars, it is very difficult to find any gold
crosses that remain on
the posts after the fluid assisted release step, and we estimate the
efficiency of release of
the particles to be greater than 99%, with less than one particle in ten
thousand remaining
on the wafer. The few bound particles that do remain are found near the edges
of the
wafer where the spin-coating of the release agent may have been adversely
affected by the
high effective contact angle introduced by the pillars. Vapor deposition of
the sacrificial
layer, rather than spin-coating, would most likely improve the release
efficiency. The
simplicity of release and the exceptional release efficiency is one of the
strengths of the.
P-DePT approach.
1000701 To continuously produce particles at a high rate, an automated system
containing the essential non-optical devices for each step in the above
process can be set
up in a continuous loop. For the example we gave, several identical pillar
templates held
in a wafer boat can be fed by an automated robotic track system into a
hexamethyldisilizane (HMDS) applicator, a spin-coater, a baker, a sputterer,
an ultrasonic
bath, a cleaning tank, a drying stage, and then back to the HMDS applicator to
complete
the loop (Figure 15). The HMDS applicator, spin coater, baker, cleaning tank
and/or a
drying system can be components of a template cleaning and preparation system
of a
system for manufacturing particles according to an embodiment of the current
invention.
The sputterer is one example of a possible deposition system for manufacturing
particles
according to an embodiment of the current invention. The deposition system is
not
limited to only a sputterer and may include other deposition systems including
those
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described in references to various examples herein. The ultrasonic bath is one
example of
a possible particle renloval system according to an embodiment of the current
invention.
However, systems for manufacturing particles according to various embodiments
of the
current invention are not limited to this specific example. The track system
is only one
possible way of performing high-throughput production. A rotating carrousel of
identical
pillar templates could also be used. Alternatively, various operations can be
performed
on only a region of the wafer as it is rotated or translated, if appropriate
deposition
methods are used. With such an automated systeni, we estimate that roughly
1011
microscale particles can be made per day per wafer without the need for human
intervention. For submicron particles, the rate of production could far exceed
1011
particles per day per wafer. By producing particles from multiple templates
simultaneously, the production rate can exceed that of particle production
methods
relying on spatially patterned radiation.
[00071] Although P-DePT is well suited for making particles that are slab-like
and
have a uniform thickness, it is also possible to make particles that have more
complex
three-dimensional shapes by appropriately modifying the surfaces of the
pillars. For
instance, it is possible to make a pillar-template suitable for creating
pyramid-shaped
particles by filling the trenches of the well-template with an inert material,
leaving the
tops of the pillars exposed, and then etching the tops of the pillars at an
angle, as can be
achieved by angular etching of an appropriately oriented polished silicon
wafer surface.
After etching, the surfaces of the pillars can be coated with a release agent.
As shown in
Figure 16, deposition of the desired particle material onto the pillars and
subsequent
release by fluid agitation yields an efficient non-optical process for
producing complex
LithoParticles. By depositing a uniform layer of the desired particle material
onto pillars
that are not flat, one can create non-planar particles that have uniform
thickness, yet
retain the contours of the tops of the pillars, as shown in Figure 17.
1000721 In addition to gold particles, we have produced plate-like square-
cross
particles made of aluminum that have thicknesses in excess of one micron,
showing that
P-DePT can be used to fabricate particle structures that are quite thick and
robust. The
ultimate limit of the particle thickness is set by the height of the pillars;
if the wells
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outside of the pillars become filled with the particle material, then the
particle material
will form a continuous interconnected layer, and no particles can be produced.
However,
if the height of the permanent pillars is larger than the lateral dimensions
of the particles,
as it is in our exaniple, then the thickness of the particles can actually
exceed the lateral
dimensions, withoiit a loss in the definition of the lateral shape. So, both
thin and thick
particles can be made using the P-DePT method.
1000731 Residual stress in the layer of deposited particle material can cause
the
particles to deform into non-planar shapes, especially when the thickness of
the deposited
layer is much less than a micron. This effect has been reported previously
(Brown,
A.B.D.; Smith, C.G.; Rennie, A.R. Fabricating colloidal particles with
photolithography
and their interactions at an air-water interface. Phys. Rev. E 2000, 62, 951-
960), but, in
our method, the gold particles remain quite planar, even after release, as can
be seen in
the optical micrographs. Further electron niicroscopy shows that the gold
particles do not
exhibit significant distortions away from planar shapes. In principle, by
depositing a thin
layer of a particle material that is known to have an inherent stress, it
could be possible to
design continuously curved particle shapes. Indeed, by relying upon stresses
created by
controlling the composition (e.g. stoichiometry) of multi-elemental particle
materials, one
can induce a desired curvature after lift-off. One can also create bilayer
deposition of two
desired particle materials that have different thermal coefficients of
expansion, yielding
two-faced Janus particles that have continuously variable shapes that can be
controlled as
a function of temperature. This can be accomplished by simply depositing a
layer of one
desired material, and then a second layer of a different desired material
having a different
coefficient of thermal expansion onto the tops of the pillars before releasing
these bi-layer
particles into a fluid.
1000741 Many different deposition scenarios, both for the sacrificial layer
and for
the target material layer, are possible once the permanent pillar template has
been made.
These materials include organics (e.g polymers), biomaterials, inorganics
(e.g. nitrides
and oxides), and metals, or combinations of any of these compositions.
Performing
multiple depositions of different desirable target materials prior to the
release step can be
used to make hybrid multi-layer particles. These deposition methods include,
but are not
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limited to, spin-coating, spray-coating, dip-coating, sputtering, physical
vapor deposition
(PVD), chemical vapor deposition (CVD), molecular beam epitaxy (MBE), and
electron-
beam metal deposition (EBMD). Directional deposition at other than normal to
the
pillar's top surface could provide a method of making particles with slanted
side-walls.
Release can be made into aqueous or non-aqueous solvents for further chemical
surface
treatment to increase particle stability against aggregation. Particle release
could take
place in any fluid, including supercritical fluids or gases, not jtist
liquids. Lastly, it may
be possible to omit the sacrificial layer if a suitable surface coating can be
used to prevent
the particles from sticking to the pillars. Such a permanent coating may take
the form of
fluorinated molecules that are attached in high density to the template
surfaces.
1000751 In P-DePT, we employ a re-usable patterned substrate with permanent
pillars and do not require exposure by any source of radiation, thereby
clearly
differentiating this approach from earlier optical approaches of Higurashi et
al.(
Higurashi, E.; Ukita, H.; Tanaka, H.; Ohguchi, O. Optically induced rotation
of
anisotropic micro-objects fabricated by surface micromachining. Appl. Phys.
Lett.1994,
64, 2209-2210), Brown, et al. (Brown, A.B.D.; Smith, C.G.; Rennie, A.R.
Fabricating
colloidal particles with photolithography and their interactions at an air-
water interface.
Phys. Rev. E 2000, 62,951-960), Harrison, Chaikin, and Mason (Sullivan, M.;
Zhao, K.;
Harrison, C.; Austin, R.H.; Megens, M.; Hollingsworth, A.; Russel, W.B.;
Cheng, Z.;
Mason, T.G.; Chaikin, P.M. Control of colloids with gravity, temperature
gradients, and
electric fields. J. Phys. Condens. Matter 2003, 15, S 11-S 18), and Hernandez
and Mason
(Hernandez, C.J.; Mason, T.G. Colloidal alphabet soup: Monodisperse
dispersions of
shape-designed lithoparticles. J. Phys. Chem. C 2007, 111, 4477-4480). P-DePT
can
offer a clear advantage of a re-usable permanently patterned template,
excellent
uniformity, and high-throughput without the complexity of optical exposure at
every
stage in the process. Because a stamping, or "imprinting" procedure (Chou,
S.Y.
Nanoimprint lithography and lithographically induced self assembly. MRS
Bulletin 2001,
26, 512; Chou, S.Y.; Krauss, P.R.; Renstrom, P.J. Nanoimprint lithography. J.
Vacuum
Sci. Tech. B 1996, 14 (6), 4129-4133; Resnick, D.J.; Mancini, D.; Dauksher,
W.J.;
Nordquist, K.; Bailey, T.C.; Johnson, S.; Sreenivasan, S.V.; Ekerdt, J.G.;
Willson, C.G.
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Improved step and flash imprint lithography templates for nanofabrication.
Microelectronic Engineering 2003, 69, 412-419), in which particles can
potentially be
stuck in wells with vertical side-walls that can inhibit facile release, is
not necessary, we
anticipate that P-DePT will be more efficient than other particle methods
involving
mechanical imprinting that we have also developed. Moreover, no special
fluorinated
surface coatings or expensive mechanical imprinting stages are required. The
internal
feature sizes and overall dimensions of the particles are not limited to the
microscale;
direct e-beam writing to a resist-coated surface or deep-UV lithography and
subsequent
etching could make templates with internal particle features, such as arm
widths on the
crosses, and overall particle lateral dimensions, smaller than 50 nm.
[000761 Figure 18 is a schematic illustration of another embodiment of P-DePT
according to the current invention. This example takes advantage of the
wetting of only
the tops of the pillars that is common when a liquid material is coated onto a
pillar
template. If the pillars on the pillar template are spaced close enough
together, many
liquids will be confined to the top surfaces of the pillars and will not
penetrate into the
troughs below. The deposition of the liquid can occur through spray-coating,
spin-
coating, dip-coating, painting, or other methods. Solidification can occur by
thermal
processes, chemical processes such as crosslinking, or through evaporation of
a carrier
solvent that may contain dispersed materials. Some advantages of this method
can
include: (1) the particle material is deposited only in the regions that will
lead to the
desired particles, so the particle material is more efficiently used, and (2)
cleaning the
substrate is easier at a later stage in the process.
1000771 Figure 19 is a schematic illustration of another embodiment of P-DePT
according to the current invention. This example takes advantage of the
wetting of only
the tops of the pillars that is common when a liquid material is coated onto a
pillar
template. If the pillars on the pillar template are spaced close enough
together, many
liquids will be confined to the top surfaces of the pillars and will not
penetrate into the
troughs below. The deposition of the liquid can occur through spray-coating,
spin-
coating, dip-coating, painting, or other methods. Depositing viscoelastic
materials such
as concentrated polymer solutions or polymer melts, on the tops of the pillars
can be
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advantageous since the elasticity inherent in the viscoelastic material can
inhibit the
formation of undesirable bi-idges of the material between adjacent pillars.
Eliniinating
liquid bridges that may occur between the top surfaces of adjacent pillars can
be achieved
by spinning the teniplate at a higher speed or by applying an external fluid
flow, acoustic
field, mechanical vibration, or electric field. Solidification can occur by
thermal
processes, chemical processes such as crosslinking, or through evaporation of
a carrier
solvent that may contain dispersed materials. Sonie advantages of this method
can
include: (1) the particle material is deposited only in the regions that will
lead to the
desired particles, so the particle material is more efficiently used, and (2)
cleaning the
substrate is easier at a later stage in the process.
1000781 Figure 20 is a schematic illustration of another embodiment of P-DePT
according to the current invention. Applying an electric field through a
voltage (i.e.
potential difference) between the fluid layer and the relief teniplate can
cause the particle
material to wet the extreme surfaces of the pillars. Solidification can occur
by thermal
processes, chemical processes such as crosslinking, or through evaporation of
a carrier
solvent that may contain dispersed materials. Some advantages of this method
can
include: (1) the particle material is deposited only in the regions that will
lead to the
desired particles, so the particle material is more efficiently used, and (2)
cleaning the
substrate is easier at a later stage in the process. This process can be
repeated to produce
bi-layer or multi-layer LithoParticles.
[00079] Figure 21 is a schematic illustration of another embodiment of P-DePT
according to the current invention. This example uses angled directional
deposition to
create LithoParticles that have non-slab shapes. Although directional
deposition is
usually along a direction parallel to the pillar axes (i.e. straight down from
the top of the
page), angled deposition, in which the direction of the motion of the
deposited material is
not aligned along the pillar axes, can also be used to create more complex
shapes. The
same kind of angled deposition could be made using well-templates, not only
pillar
templates.
1000801 Figure 22 is a schematic illustration of another embodiment of P-DePT
according to the current invention. This example takes advantage of the
wetting of only
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the tops of the pillars that is comnion when a liquid niaterial is coated onto
a pillar
template. In this particular example, only two different particle materials
have been
added to the tops of the pillars in sequence to create bi-layer
lithoparticles. This
procedure can be extended to add additional layers of the same or different
particle
materials to build up multi-layer lithoparticles. Liquid deposition to the
tops of the pillars
is just one way to create the particles; other forms of deposition in a
desired sequence
could be used to create and customize additional layers of different types of
materials that
fonn the particle material.
[000811 Figure 23 is a schematic illustration of another embodiment of P-DePT
according to the current invention. In this example, microscale particles
(e.g. polystyrene
spheres, silica spheres, clay), nanoscale particles (e.g. iron oxide, quantum
dots,
dendrimers), and molecular species (e.g. star polymers, plasticizers,
proteins,
polypeptides, dyes) can be incorporated into the matrix of the particle
niaterial to form a
customized complex composition.
[000821 Figure 24 is a schematic illustration of another embodiment of P-DePT
according to the current invention. Rather than using fluid-assisted release
(with or
without agitation), as in some of the other examples described, the
LithoParticles are
released from the substrate by changing the temperature. As an example, the
release
material could consist of a solid over a range of temperatures used to form
particles;
subsequently, the temperature is changed out of this range to cause the
release layer to
become fluid, thereby liberating the lithoparticles from the substrate.
Optionally, this
approach could be done with or without the presence of a fluid into which the
lithoparticles would be dispersed.
Further Embodiments
[00083) Both pillars and wells can be made on the same template surface to
yield a
mixed template that can produce particles by both processes. For this kind of
mixed
template, the same deposition step can create discrete disconnected regions in
the form of
the desired particles on the tops of the pillars and in the bottoms of the
wells
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simultaneously. A single lift-off step can release the particles both from the
pillars and
from the wells.
1000841 More generally, the solid template can be created in such a nianner as
to
provide several different plateau levels at different depths from its topmost
surface upon
which the desired material can be deposited. The desired material can be
deposited in a
manner that leaves disconnected regions of this material at different levels
in the form of
the desired particle shapes. These disconnected regions can be released from
the
template, yielding particles in solution. In principle, using this approach,
all of the
deposited nlaterial can be used to form desired particles without waste,
provided the
different shapes can be formed on the template at different levels and
completely fill the
available surface area. This would be a highly efficient implementation that
would make
excellent use of the deposited material. The example in Figure 25 shows a
template for
making square shaped particles comprised of six different levels that are
arranged in a
pattern so that no two levels of the same height are neighbors when repeated
everywhere
over the surface of the template. Each of the levels could have additional
surface features
that can be used to create texturing, asperities, bonding sites, or
indentations on the
surfaces of the particles that are produced. Directional deposition of the
desired particle
material from above onto this template will result in identical square
particles that are
disconnected from each other being produced over the entire surface of the
template. A
single release step can release the particles from all of the different levels
simultaneously,
and the template can be re-used. In general, the profile of the top surface
can be of a
shape other than a square (e.g. square crosses, Penrose tiles, etc.) could be
used. Several
different shapes can be tiled at different levels onto the same template. The
simple
example for squares in Figure 25 illustrates the more general type of template
that can be
used to make particles by a template deposition process.
1000851 Templates can potentially have many different forms other than being
niade on a flat wafer surface. The overall template surface does not have to
be flat for
either the pillar deposition templating process or the well deposition
templating process
in order to produce useful particles. For instance, a template can be made on
a curved
surface, such as a cylinder, which could be spun to expose different portions
of the
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cylinder to cleaning, deposition, and release processes. Using such a curved
template that
has appropriate pillars and/or wells on the surface, one niay be able to
optimize the
processing steps into a continuous particle production device that does not
require
repeated exposure with radiation. Templates made from flexible solid materials
could be
adhered to a solid surface. Well templates could potentially be made by
niaking a thin
porous film of a flexible solid material that has holes of the desired
particle shape and
then adhering this film to a non-porous solid support. Indeed, lifting off the
top
contiguous layer of the simple well deposition templating process could
potentially
produce a film that could be used, in turn, to make another well template if
this film is
deposited and bonded to a solid support.
1000861 Templates can be made by many different possible procedures. Standard
lithography procedures, such as electron beam lithography and optical
lithography, can be
used in conjunction with etching, to make the templates. However, other
methods can be
used, too. One method involves coating a wafer surface with diblock polymers
that form
phases of dots or short stripes that can be etched onto the wafer's surface to
provide either
pillars or wells in the form of the dots or stripes. Another possible method
is to coat the
wafer surface with a solution of polymer particles and use these particles as
a mask
during an etching process. This type of process could be used to make circular
pillars or
even ring-like pillars. If complex particle shapes, such as those made using
lithographic
methods, are deposited, templates for reproducing their shapes could
potentially be made
this way. Yet another method of making a template could be to cover a wafer's
surface
with a microporous or nanoporous membrane or film. This kind of well template
may
not be comprised of only one material but may be made instead from two or more
materials that have been put together to create the desired pillars and wells.
Optionally,
the exposed surface of the wafer could be selectively etched using an ion
etcher in the
regions where the holes appear and the membrane could then be removed from the
surface.
1000871 Multiple deposition steps using different materials can be used in
combination with templates in order to make conlplex particles that have
layers of
different kinds of materials, including organics, inorganics, metals, alloys,
and
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biomaterials. By combining sequences of deposition of different desired
materials in
controlled amounts with complex templates that have multiple levels in
different shapes,
it is possible to produce very complex particles that have differently shaped
substructures
of particularly desired materials located in pre-specified regions. In
particular, selective
spatially patterned deposition can be used in combination with the templates
to create
local sites for producing pre-specified interactions, whether attractive or
repulsive,
between different particles. Alternatively, local regions on the surfaces of
the particles
can be made rough through a selective deposition process that coats only part
of the
particles' surfaces with a desired material in a manner that produces an
enhanced surface
roughness in a desired sub-region of the particle. Thus, by controlling the
deposition as
well as the template, it is possible to design particles that have customized
localized
surface coatings that can interact with local sites on the surfaces of other
particles to form
assemblies of particles that have either the same or different shapes.
[00088] Before the particle is separated, it typically will be or will become
at least
partially solid so that it retains a geometrical feature of the surface
portion of the template
(or coated template) that it was in contact with, after the separation. The
forming of a
particle could involve depositing a liquid dispersion and then inducing a
chemical
reaction, thermal polymerization of a polymer component, photo-induced
polymerization,
plasma-induced polymerization, sintering, a crosslinking reaction, a gelation,
an
evaporation of the solvent, an aggregation or agglomeration of materials, a
jamming, an
entanglement, a denaturation, and/or a bonding.
[00089] The constituent material as first applied to the template can be a
vapor, a
liquid, or a solution, for example. The maximum dimension associated with any
of the
components contained within the constituent material should be smaller than
the
maximum dimension associated with the portion of the surface for creating the
particles.
For example, it may not be reasonable to coat the surfaces of the pillars with
giant
particles that are larger than the pillars themselves.
(00090] The structured substrate can be produced from a flat smooth substrate
by a
lithographic process involving at least one of electron-beam lithography,
optical
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lithography, ultraviolet lithography, dip-pen lithography, x-ray lithography,
imprinting,
stamping, deposition, patterning, and etching.
100091] The current invention is not limited to the specific embodiments of
the
invention illustrated herein by way of example, but is defined by the claims.
One of
ordinary skill in the art would recognize that various modifications and
alternatives to the
examples discussed herein are possible without departing from the scope and
general
concepts of this invention.
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