U.S. patent application number 09/946115 was filed with the patent office on 2002-02-21 for designer particles of micron and submicron dimension.
This patent application is currently assigned to Washington University. Invention is credited to Ruoff, Rodney S..
Application Number | 20020022124 09/946115 |
Document ID | / |
Family ID | 25528004 |
Filed Date | 2002-02-21 |
United States Patent
Application |
20020022124 |
Kind Code |
A1 |
Ruoff, Rodney S. |
February 21, 2002 |
Designer particles of micron and submicron dimension
Abstract
Micron-sized particles are produced in quantity by one of
various methods, including generally the steps of preparing a
substrate surface through a lithographic process, the surface being
characterized by defining a plurality of elements, depositing a
layer of particle material on the substrate surface including the
elements, processing the substrate surface to isolate the material
deposited on the elements, and separating the particles from the
elements. The size and shape of the elements predetermine the size
and shape of the particles. The elements may comprise, inter alia,
pillars of photoresist or spaces on the substrate surrounded and
defined by photoresist.
Inventors: |
Ruoff, Rodney S.; (Clayton,
MO) |
Correspondence
Address: |
HOWELL & HAFERKAMP, L.C.
Suite 1400
7733 Forsyth Boulevard
St. Louis
MO
63105
US
|
Assignee: |
Washington University
|
Family ID: |
25528004 |
Appl. No.: |
09/946115 |
Filed: |
September 4, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09946115 |
Sep 4, 2001 |
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08980980 |
Dec 8, 1997 |
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6284345 |
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Current U.S.
Class: |
428/325 ;
428/195.1; 428/331; 430/270.1 |
Current CPC
Class: |
Y10T 428/24802 20150115;
Y10T 428/24372 20150115; Y10T 428/259 20150115; B81C 2201/034
20130101; Y10T 428/25 20150115; Y10T 428/24893 20150115; C23C
14/042 20130101; G03F 7/00 20130101; B81C 99/008 20130101; C23C
14/0005 20130101; Y10T 428/252 20150115; Y10T 428/2982
20150115 |
Class at
Publication: |
428/325 ;
428/331; 428/195; 430/270.1 |
International
Class: |
B32B 003/00; G03C
001/76 |
Claims
What is claimed is:
1. An array comprised of a plurality of discrete particles on a
substrate, said particles being separable from said substrate and
between about 0.1 microns and about 25 microns in width
2. The array of claim 1 wherein said particles are substantially
uniformly sized and substantially uniformly shaped.
3. The array of claim 2 wherein the particles are separable from
said substrate by removing that portion of said substrate which
attaches each of said particles to said substrate.
4. The array of claim 3 wherein said removable portion of said
substrate is removable by dissolving said substrate.
5. The array of claim 2 wherein the particles are separable from
said substrate by vibration.
6. The array of claim 2 wherein the particles are separable from
said substrate by mechanically separating said particles from the
portion of substrate which attaches each of said particles to said
substrate.
7. The array of claim 2 wherein said particles comprise at least
one deposited layer of a desired particle forming substance on said
substrate.
8. The array of claim 2 wherein said substrate is multi-level, with
the particles being formed on only one of the levels of said
multi-level substrate.
9. The array of claim 8 wherein said particles are formed on a top
level of said substrate.
10. The array of claim 8 wherein said particles are formed on a
level of said substrate below a top level of said substrate.
11. The array of claim 2 wherein said particles are comprised of a
plurality of layers of different materials.
12. The array of claim 2 wherein said particles are comprised of
one or more materials chosen from the group: metals, insulators,
semiconductors, ceramics, and glasses.
13. The array of claim 11 wherein each of said layers is a
deposited layer of material.
14. The array of claim 2 wherein each of said particles is
magnetizable.
15. The array of claim 2 wherein each of said particles is
magnetized.
16. The array of claim 2 wherein each of said particles is
disk-shaped.
17. The array of claim 2 wherein each of said particles has an
internal surface area when separated from said substrate.
18. The array of claim 17 wherein said internal surface area
comprises a generally circular opening in said particle.
19. The array of claim 2 wherein said particles are substantially
uniformly spaced across a surface of the array.
20. The array of claim 19 wherein said substrate comprises a wafer,
said wafer including a layer of resist, said particles being
layered on top of said resist layer.
21. The array of claim 20 wherein said layer of resist comprises a
plurality of separated elements each of said elements being of a
size and shape as desired for each of the particles.
22. The array of claim 21 wherein said elements are the residue
from a lithographic process.
23. The array of claim 22 wherein said elements are the residue
from a photolithographic process.
24. The array of claim 23 wherein said elements are soluble in
solutions for which the particles are non-soluble so that the
particles are thereby separable from said substrate.
25. A silicon wafer substrate includes a layer of photoresist
residue comprised of a plurality of substantially uniformly sized,
substantially uniformly shaped elements spaced substantially
uniformly across at least a portion of a surface of the substrate,
each of said elements having a particle layered on top thereof, and
said photoresist being soluble in solutions for which the particles
are non-soluble so that the particles are thereby separable from
said substrate by immersion in a solution.
26. The silicon wafer of claim 25 wherein said particles are
multi-layered with different layers of dissimilar deposited
material forming said layers.
27. The silicon wafer of claim 26 wherein the particles are between
about 0.1 microns and about 25 microns in width.
28. A silicon wafer substrate includes a layer of photoresist
residue comprised of a pattern defining a plurality of
substantially uniformly sized, substantially uniformly shaped
elements spaced substantially uniformly across at least a portion
of a surface of the substrate, each of said elements having a
particle layered on top thereof, and said photoresist being soluble
in solutions for which the particles are non-soluble so that the
pattern is thereby separable from said substrate by immersion in a
solution, leaving the particles in place on said elements.
29. The silicon wafer substrate of claim 28 wherein the particles
are between about 0.1 microns and about 25 microns in width.
30. The silicon wafer substrate of claim 29 wherein said particles
are multi-layered with different layers of dissimilar deposited
material forming said layers.
31. A silicon wafer substrate includes a sacrificial layer of a
first material upon which is deposited a layer of particle
material, the particle material layer having a layer of photoresist
residue comprised of a plurality of substantially uniformly sized,
uniformly shaped elements spaced substantially uniformly across at
least a portion of a surface of the substrate to protect a
similarly sized and shaped particle thereunder during an etching
process which removes the surrounding portion of particle layer to
thereby form a plurality of particles on said substrate, said
particles being separable from said substrate by dissolving the
sacrificial layer.
32. The silicon wafer substrate of claim 31 wherein the particles
are between about 0.1 microns and about 25 microns in width.
33. The silicon wafer substrate of claim 32 wherein said particles
are multi-layered with different layers of dissimilar deposited
material forming said layers.
34. A method for forming a plurality of particles having a
predetermined shape and a size between about 0.1 microns and about
25 microns, said method comprising the steps of: a. preparing a
substrate, b. depositing at least one layer of a particle material
on said substrate, and c. separating the particles from said
substrate.
35. The method of claim 34 wherein the step of preparing the
substrate includes the step of patterning the substrate with a
lithographic process.
36. The method of claim 35 wherein the step of depositing the
particle layer material includes the step of metal deposition.
37. The method of claim 36 wherein the step of preparing the
substrate includes the step of patterning a wafer.
38. The method of claim 37 wherein the step of patterning the wafer
includes the steps of applying a layer of photoresist to a base,
and processing the photoresist to create a pattern on said base to
define the surfaces for receiving the deposited layer of particle
material.
39. The method of claim 38 wherein the step of processing the
photoresist includes the steps of preparing a mask and exposing the
layer of photoresist through the mask.
40. The method of claim 39 wherein the step of separating the
particles includes the step of dissolving the photoresist which
attaches the particles to the base.
41. The method of claim 40 wherein the step of separating the
particles includes the step of vibrating the wafer.
42. The method of claim 39 wherein the step of separating the
particles includes the step of vibrating the wafer.
43. The method of claim 39 wherein the step of preparing a mask
includes the step of preparing a mask which creates a pattern of
pillars of photoresist which define the size and shape of the
particles.
44. The method of claim 39 wherein the step of preparing a mask
includes the step of preparing a mask which creates a pattern of
photoresist that defines a plurality of spaces in said photoresist
which define the size and shape of the particles.
45. A method for making a plurality of particles, each of said
particles having a predetermined size between about 0.1 microns and
about 25 microns, and a predetermined shape, and said particles
being substantially uniformly sized and substantially uniformly
shaped, the method comprising the steps of: preparing a substrate
comprising a base with a layer of photoresist thereon, said
photoresist being patterned to form a plurality of elements which
define the size and shape of the particles, depositing at least one
layer of particle material on said substrate, said elements
receiving said particle material to thereby form said particles,
and removing said particles from said substrate.
46. The method of claim 45 wherein said photoresist forms the
elements and the particle material is deposited thereon to form
said particles.
47. The method of claim 45 wherein said photoresist surrounds and
defines said elements and the particle material is deposited on
said base.
48. A method for making a plurality of particles, each of said
particles having a predetermined size between about 0.1 microns and
about 25 microns, and a predetermined shape, and said particles
being substantially uniformly sized and substantially uniformly
shaped, the method comprising the steps of: preparing a substrate
comprising a base, depositing a sacrificial layer on said base,
depositing at least one layer of particle material on said
substrate, depositing a layer of photoresist on said layer of
particle material, said photoresist being patterned to form a
plurality of elements which define the size and shape of the
particles, removing the particle layer surrounding the pattern of
photoresist to thereby form the particles under the remaining
photoresist, and removing said particles from said substrate.
49. The method of claim 48 wherein the step of removing the
particles includes the step of dissolving the sacrificial
layer.
50. The method of claim 49 wherein the step of removing the
particle layer surrounding the pattern of photoresist includes the
step of etching the exposed particle layer.
51. The method of claim 50 wherein the step of removing the
particles includes the step of vibrating the substrate.
52. An array comprised of a plurality of discrete particles on a
substrate, said particles being between about 0.01 microns and
about 0.1 microns in width.
53. The array of claim 52 wherein said particles are substantially
uniformly sized and substantially uniformly shaped.
54. The array of claim 53 wherein the particles are separable from
said substrate by removing that portion of said substrate which
attaches each of said particles to said substrate.
55. The array of claim 54 wherein said particles comprise at least
one deposited layer of a desired particle forming substrate on said
substrate.
56. A method for forming a plurality of particles having a
predetermined shape and a size between about 0.01 microns and about
0.1 microns, said method comprising the steps of: a. preparing a
substrate, b. depositing at least one layer of particle material on
said substrate, and c. separating the particles from said
substrate.
57. The method of claim 56 wherein the step of preparing the
substrate includes the step of patterning the substrate with a
lithographic process.
58. The method of claim 57 wherein the step of depositing the
particle layer material includes the step of metal deposition.
59. The method of claim 58 wherein the step of preparing the
substrate includes the step of patterning a wafer.
60. The method of claim 59 wherein the step of patterning the wafer
includes the steps of applying a layer of photoresist to a base,
and processing the photoresist to create a pattern on said base to
define the surfaces for receiving the deposited layer of particle
material.
Description
BACKGROUND AND SUMMARY
[0001] Small particles, i.e. particles approaching one micron or
less, are known in the art. These particles are made with various
techniques and may be comprised of widely varying materials. For
example, particles may be made of gold from colloidal gold
solutions, tungsten from a process involving grinding, sifting, and
filtering, and still other lesser used materials such as stainless
steel, frozen water, and plastic spheres. There are still other
similarly sized particles made from other materials as well.
However, all of these particles produced by these various methods
share certain characteristics. For example, the inventor is unaware
of any particles, or process for producing particles, which have a
uniform size and shape regardless of whether there is an
opportunity to choose a particular shape. For example, many
processes produce particles which are essentially globular, but
those globular shapes vary from particle to particle and also with
respect to their size. Still other processes produce particles
which have irregular shapes and with particles having different
shapes within the same yield. Many of the processes have a
significant range in particle size with some of these processes
producing particles having less than a smooth distribution in
sizes. In other words, there is not a consistent number of
particles of each particle size contained within a harvest of any
particular process. Furthermore, some particle materials and
processes are not capable of being produced in all sizes. Still
another limitation in the prior art is that the kinds of materials
which may be utilized are process dependent. In other words,
certain types of metal may not be used to produce particles through
the colloidal solution process due to the chemistry.
[0002] For illustrative purposes, the inventor will now describe
one particular use of micron-sized particles. These are used in
implementing a technology known as biolistics. With this
technology, inert or biologically active particles are propelled at
cells at a speed whereby the particles penetrate the surface of the
cells and become incorporated into the interior of the cells. The
process can be used to mark cells or tissue or to biochemically
affect tissues or tissue in situ as well as single cells in vitro.
There are various kinds of apparatus used to propel the particles
into the cells, examples of which are disclosed in U.S. Pat. No.
5,371,015; U.S. Pat. No. 5,478,744; and U.S. Pat. No. 5,179,022,
the disclosures of which are incorporated herein by reference.
These patents also disclose various uses of the micron-sized
particles in the area of biolistics. These uses include gene
therapy for the correction of genetic disorders by expressing
healthy versions of the defective gene, genetic immunization for
eliciting immune responses against specific antigen after
inoculating cells with the DNA encoding the antigen, genetic
engineering of animals for producing new and useful phenotypes, the
determination of functions of genes in an in-vivo setting, and
cancer therapy for introducing therapeutic genes into tumorous
cells. Again, these uses are only exemplary as biolistics is a
relatively new and evolving technology.
[0003] As might be expected, it would be desirable in implementing
biolistics for a technician to be able to choose both a particle's
shape as well as its size and be ensured that a collection of these
particles would be uniformly shaped and uniformly sized in order
that a uniform effect may be expected upon their use. Furthermore,
the particles may be particularly shaped in order to enhance the
particular application desired to be implemented. One such example
would be to provide particles having an interior surface, much like
a donut-shaped particle, so that the interior surface may be filled
with a biologically active material desired to be delivered into
the cell. Typically, in the prior art as known to the inventor,
particles are coated with the biologically active materially and as
might be expected some of this biologically active material is lost
as the particles are propelled and injected into the cells. This
happens through abrasion, acceleration, etc. of the particle's
surface as it is delivered.
[0004] To the extent that the particle size, shape, and other of
its properties can be controlled, new uses for some micron
particles may be considered. For example, controlling the
particle's size and rendering it magnetizable permits consideration
of the particles' use for reliable and safe transportation through
a patient's blood system to a desired site with a magnetic field
gradient and under computer control. Still other new uses may be
considered and are limited solely by the ingenuity of the scientist
or engineer.
[0005] Also known in the prior art are substrates having arrays of
sub-micron sized metal deposits. For example, nanometer size
platinum particle arrays were prepared by electron beam
lithography. The Pt particles were 50 nm in diameter and spaced 200
nm apart on an oxidized silicon wafer. See P. W. Jacobs, et al.,
Surface Science, 372, L249-L253 (1997). Another example of e-beam
patterning was the preparation of two-dimensional arrays of
amorphous R-Co (R=Sm and Gd) square particles on 20 nm thick
niobium films. See O. Geoffrey, et al., Journal of Magnetism and
Magnetic Materials, 121, 223-226 (1993). A third example involved
the deposition of Ni.sub.80Fe.sub.20 boxes with width and spacing
of 1-.mu.m thick on a PMMM resist film, followed by liftoff, which
resulted in the production of "box" arrays of 50-nm thick
Ni.sub.80Fe.sub.20 boxes with width and spacing of 1 .mu.m. See A.
Maeda, et al., Journal of Applied Physics, 76(10), 6667 (1994). A
further example is the production of ultra-small particle arrays by
high resolution electron beam lithography, in which arrays of
silver and gold-palladium particles smaller than 10 nm in diameter
and center-to-center spacings as low as 25 nm were made. See H.
Craighead, et al., Journal of Applied Physics, 53(11), 7186
(1982).
[0006] Other methods of making metal particle arrays include by
"nanosphere lithography" where uniformly sized latex spheres are
deposited onto a substrate such that they closest-pack; metal
deposition with liftoff results in, for example, triangle shaped
particles on a hexagonal lattice. See J. Hulteen, et al., Journal
of Vacuum Science and Technology, A 13(3), 1553 (1995). Another
approach for making small metal particles is by fabricating them
with a scanning tunneling microscope. In one approach, Fe(CO).sub.5
is decomposed by the tunneling electron beam, which results in the
deposition on the substrate of small iron deposits with approximate
diameter of 25 nm. See A. D. Kent, et al., Journal of Applied
Physics, 76(10), 6656 (1994).
[0007] Nanoimprint lithography has been used to create metal
patterns with feature size of 25 nm and spacing of 70 nm;
compression imprinting followed by liftoff of a metal deposited
layer results in the 25 nm particles on the substrate. See S. Y.
Chou, et al., Science, 272, 85 (1996). These substrates were used
with the deposits secured to the substrate and the inventor is
unaware of any teaching or suggestion in the prior art that these
deposits could be separated from the underlying substrate to
produce discrete particles.
[0008] To salve these and other problems in the prior art, the
inventor herein has succeeded in designing and developing a method
of producing micron and submicron particles having a uniform
pre-selected shape and size, as well as the particles themselves.
With the inventor's process, the composition of each particle, its
physical properties and chemical properties, may all be
pre-selected or "designed" as desired to satisfy a particular need
of the designer. The particles may be made from virtually any
material amenable to deposition layering techniques, various
different shapes, except perhaps for spheres or globular-shaped
particles, of multi-layered construction from dissimilar materials,
and engineered to exhibit desirable physical and chemical
properties after formation.
[0009] Generally, the method of the present invention includes the
steps of preparing a substrate and, more particularly, a surface on
the substrate for receiving a layer of particle material. This
preparation process includes a lithographic patterning of a surface
of the substrate with any suitable lithography process. As
explained more specifically in the preferred embodiment, the
inventor has utilized photolithography including layering the
substrate with photoresist and then exposing the substrate through
a mask whose pattern is created using a CAD process. However,
e-beam lithography, imprint lithography, x-ray lithography, or
other kinds of lithographic processes as known in the art may be
used as well. After the surface of the substrate is prepared, a
layer of material is deposited on the substrate using any
appropriate metal deposition process such as vapor deposition,
sputter deposition, CVD deposition, or electro-deposition. One or
more layers of particle material may be deposited, and the layers
may be of the same or dissimilar materials so as to make layered or
sandwich type particles. The last step in the process involves
separating the particles from the substrate which, depending upon
the particular process utilized, may include emerging the substrate
in a solvent, vibrating the substrate such as by sonification, or
chemical etching, or any other suitable such process. The particles
may then be collected and washed thoroughly in order to ready the
particles for further use.
[0010] The shape, size, and uniformity of the particles is
determined and controlled in the lithographic step of preparing the
substrate surface. As explained more completely in the detailed
description of the preferred embodiment which follows, and in the
event that photolithography is utilized, the photo mask pattern
helps to determine these parameters. After its preparation, it is
used to mask a light exposure for partially burning away a layer of
photoresist to create elements for receiving the deposited layers
of metal forming the particles. Therefore, it is important to
prepare the mask with as accurate an image as is possible to ensure
sharp lines and corners (if the particle shape so requires) so that
the particles may be shaped and sized as desired.
[0011] While several advantages and features of the present
invention of a process for making submicron-sized particles and the
particles themselves have been explained, a more thorough
understanding of the invention may be attained by referring to the
drawings attached hereto and by studying the detailed description
of the preferred embodiment which is provided for illustrative
purposes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIGS. 1A to C are views of illustrative shapes which the
micron sized particles of the present invention may take.
[0013] FIG. 2A to D are views of the metal deposition step of the
process after the substrate surface has been prepared with a
lithographic process.
[0014] FIG. 3A to E are several views of an imprint process for
preparing the substrate.
[0015] FIG. 4A to D illustrate still another alternative process
for forming the particles of the present invention.
[0016] FIG. 5A to D illustrate still another alternative process
for forming the particles of the present invention.
[0017] FIG. 6 is an electron microscope image depicting a particle
adhered to an element on a substrate.
[0018] FIG. 7 is an electron microscope image illustrating a
plurality of particles adhered to elements on a substrate.
[0019] FIG. 8 is an electron microscope image illustrating a
plurality of particles separated from the substrate.
[0020] FIG. 9 is an electron microscope image illustrating a
plurality of uniformly sized and shaped particles formed through
the process of the present invention.
[0021] FIG. 10 is an electron microscope image illustrating several
particles impacted into a surface as would be achieved in a
biolistic particle application.
[0022] FIG. 11 is an electron microscope image similar to FIG. 10
except at greater magnification.
[0023] FIG. 12 is an electron microscope image illustrating a
particle of the present invention pancaked onto a surface as would
be experienced in a biolistic particle application.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0024] The present invention includes a process for forming
micron-sized particles having a diameter of between about 0.1
microns and about 25 microns which are of uniform shape and size
and which can be produced in relatively large numbers in a single
pass through the process. It is contemplated by the inventor that
particles even as small as 0.1 micron may be made with one or more
of the methods disclosed herein, and in great number. As
illustrated in the examples given herein, 10.sup.8.times.10.sup.8
particles having substantially the same shape and size may be
produced all at the same time. The particle's shape may be as shown
in FIGS. 1A to C and may include a disk 20, a regular polygon such
as a hexagon 22, an annulus 24 with a central interior surface
suitable for receiving and carrying any desired material, a
double-pointed oblong particle 26, the same double-pointed oblong
particle as shown at 26 except comprising multiple layers of
dissimilar materials as at 28, and a flattened single-pointed
particle 30. The particle shapes and sizes 20-30 are illustrated
herein as examples, but they are not intended to be exhaustive. It
is noted that many of the particles 22-30 are flattened although
this is not necessarily the case. Other particle shapes and sizes
may be utilized and are limited only by the imagination of the
designer and the physical limitations of the processes used to
prepare and coat the substrate.
[0025] In one embodiment, the process of the present invention is
illustrated in its various steps as shown in FIG. 2A to D. This
process, and the other alternative processes, are more specifically
described in the examples given herein. However, as an aid in
understanding the overall invention, these various alternative
processes will now be generally described. Again, with respect to
FIG. 2, a substrate is first prepared and may consist of a layer of
silicon 34 with a layer of photoresist 36 applied to a surface
thereof. A mask 38 is next prepared with a pattern generated by a
computer aided design (CAD) program. A data file is created for
controlling an optical pattern generator which includes parameters
for defining the particle shape, size and number. The pattern
generator uses this file in creating an appropriate image for
exposing the photoresist layer 36. The mask 38 may be a piece of
glass coated with a thin layer of chromium and photosensitive
polymer, i.e. photoresist. The mask 38 may either be prepared for
use with positive photoresist or negative photoresist. As shown in
FIG. 2, a positive photoresist is used so that the particles are
formed on elements defined by surrounding photoresist (liftoff). In
experiments, the inventor has found a positive photoresist may be
desirable as it can contribute to greater resolution resulting in
particles of smaller size and sharper edges in shape. After the
mask 38 is used to expose the photoresist through UV light, the
substrate was readied for deposition by developing and washing
processes. A layer 40 of particle material was next deposited on
the substrate including the portion of substrate created as holes
in the photoresist 36 on the silicon layer 34. These elements 42
form the places at which the particles 44 are actually formed. The
photoresist 36 is removed from the silicon layer 34 by, for
example, being soaked in a solvent to dissolve the photoresist, and
the particles 44 then may be separated through further soaking in
solvents to dissolve a sacrificial layer (not shown) between the
particles and the silicon layer 34. Thus, in implementation of the
process of the present invention, a large number of particles of
uniform shape and size are formed on a substrate and separated from
the substrate, and harvested.
[0026] There are a variety of ways to pattern surfaces, and (as per
this invention) to dislodge particles from them. To have a common
term for the surface region upon which the particle resides prior
to being dislodged, this is defined and referred to as the
"element". This is not to be confused with chemical elements (the
atoms). For example, if a pattern is created which has 800,000,000
pillars of 1 micron diameter, and the particles are formed on the
pillars, then the pillar is the element. This can be the situation
when a negative photoresist is used, or a reverse-image process, as
is discussed in one of the specific examples below. In contrast, if
a positive resist is used and developed, the elements would be the
patterned substrate regions of the substrate, onto which the
photoresist had originally been spun-coat (or in the case of
imprint lithography, a polymer film is spun-coat which is not
necessarily a photoresist). Metal would be deposited, the polymer
layer removed, and an array of identical particles, each residing
on an "element", would be present. The particles would then be
removed from the substrate (which can be a sacrificial layer and
for example dissolved with a solvent which does not etch the
particles; or could be a non-stick surface so that the particles
could be shaken loose, or pulled off with an adhesive surface such
as scotch tape). Each "element" is the region below the particle
prior to dislodging.
[0027] An alternative method may be utilized as shown in FIG. 3. As
illustrated in FIG. 3A, an imprinter 46 is used with a silicon
substrate 48 in this process. As shown in FIG. 3B, a mold release
compound is applied to the imprinter 46 and an adhesion promoter 50
is applied to the silicon substrate 48. As shown in FIG. 3C, a
polymer resin 52 is applied over the adhesion promotor 50. The
imprinter 46 may then be heated and used to imprint its pattern
into the polymer resin 52 as shown in FIG. 3D. With some materials,
heating is not required. As illustrated in FIG. 3E, the mask 46 is
withdrawn from the substrate leaving behind a pattern defining
elements which may be further prepared prior to deposition of a
layer of particle material by plasma cleaning. Metal would then be
deposited. At the completion of this process, the size and shape of
the particles has been determined by the size and shape of the
elements 51. Again, as illustrated in FIG. 3, the elements 51 have
been defined in terms of the border surrounding tit with polymer
resin 52. This same patterning approach may be utilized except that
the elements are formed at the top of the polymer resin 52.
[0028] Still another alternative process for forming particles of
the present invention include that which is disclosed in FIG. 4A to
D. As shown in Figure A, a patterned conducting material 56 is
applied to a silicon substrate 58. The pattern includes a
particle-sized trough 60 which may be formed by any convenient
means. A thin layer of sacrificial liner material 62 may be applied
to the particle forming surface sidewalls such as by
electroplating. A particle material layer 64 may be deposited on
top of the sacrificial layer 62 such as by electrocoating or
electroplating or other alternative means. The sacrificial layer 62
is then dissolved or otherwise removed to separate the particle 66
from the substrate 56.
[0029] Still another alternative process is illustrated in FIG. 5A
to D and includes a silicon substrate 68 having a silicon dioxide
layer 70 applied thereto with the plurality of particle trenches 72
carved therein or otherwise patterned therein using any appropriate
method such as lithography, etc. A non-stick surface 74 may be
conveniently applied across the entire patterned silicon dioxide
layer 70. A layer of particle material 76 may be continuously
applied and retained in the particle trough 72 by any convenient
means including flowing the particle material 76 into the particle
troughs 72. After the particle 78 has formed, it may be separated
from the substrate 70 by vibration, centrifuge, or any other
suitable means.
[0030] To further illustrate the present invention, the inventor
has formed particles utilizing essentially the process illustrated
in FIG. 2 except that a positive photoresist, reverse image process
was utilized. An explanation of the general methodology and
specific examples is now provided as an aid to further understand
the present invention.
[0031] The particles of the present invention may be fabricated by
applying a combination of photolithography, metal deposition and
etching techniques. Using this approach, circular shaped flat
particles (disks) have been prepared of the following materials:
Cu, W, and sandwiches Cu/Ni/Cu, Si/Au/Si. The process of particle
fabrication (except W particles) can generally be described in four
steps:
1 I. Mask preparation II. Wafer patterning III. Metal deposition
IV. Lift off
[0032] Particle fabrication started with a mask whose pattern was
created using computer aided design (CAD). A file was created which
controlled the optical pattern generator. The file contained the
parameters which defined the particle shape, size, and number. The
pattern generator created the appropriate images and exposed the
photoresist mask. The mask began as a piece of glass coated with a
thin layer of chromium and photosensitive polymer, that is,
photoresist. Depending on photoresist type, exposed or unexposed
areas were removed using chemical development. The developed areas
were removed to expose some areas of the chromium. Depending on the
particle number and configuration, it was necessary to fabricate a
secondary mask. Items such as cost of the mask production and the
time dictated this step.
[0033] The next step was wafer patterning. Silicon wafers were
coated with photoresist, exposed by UV light through the mask, and
developed. The procedure of coating wafers with photoresist
included cleaning the surface, priming it, and spinning photoresist
onto it. Depending on the photoresist types these prepared wafers
were baked before or (and) after light exposure to evaporate
excessive solvent. Wafers were primed by washing them in acetone
and isopropanol and applying primer to make their surface
hydrophobic and prevent moisture from collecting. Photoresist was
dripped from a pipet onto the wafer surface while it was spinning
at high speed to spread photoresist over the surface (spin
coating). The uniformity of the photoresist layer was critical for
subsequent photolithography processing. Variations in photoresist
thickness should not exceed 5-10 nm. The wafers were exposed using
a 5-.times. or 10-.times. stepper Projection Mask Aligner. The
image reversal process was used to obtain the negative structure
using a positive photoresist. Contrary to negative photoresists,
the positive one allowed higher resolution to be achieved (as a
result, particles of smaller size and controllable shape were
produced). Carboxylic acid was produced as a result of a
light-assisted reaction in the photoresist, which increased the
polymer solubility by a factor of ten. High temperature treatment
in an ammonia environment was used to neutralize the carboxylic
acid in the exposed areas of photoresist, thus making them poorly
soluble and non-photosensitive. Subsequent flood UV exposure with
contact aligner and development made the negative image on the
wafer (leaving the initially exposed areas). Characterizing the
patterned wafer with light and scanning electron microscopes was
necessary to correct some stepper parameters like exposure time and
focus settings, and thus obtaining good settings for these
parameters. After washing and drying, the wafer was ready for the
next steps. Fabrication of the particles with sharp corners
required applying the optical proximity correction (OPC) method.
This method is based on modifying the mask so that a uniform light
intensity distribution of the bulk of the photoresist is achieved.
By applying the OPC technique it should be possible to minimize the
corner radius down to 100-200 nm on 0.5-1 .mu.m size features.
[0034] Metal deposition was done with thermal or electron beam
evaporation, sputtering, or electro-deposition. A variety of metals
were deposited. The reasons for choosing one method or another are
described for each case.
[0035] To lift off the particles the wafer was soaked in the
appropriate organic solvent to remove the photoresist. Gentle
sonication was required in some cases. W particles are to be lifted
off by dissolving an aluminum sacrificial layer in the aluminum
etchant. Centrifugation was used to separate the particles from the
suspension.
EXAMPLE 1
[0036] Disk shaped Cu particles of 1 .mu.m diameter and 200 nm
thickness were fabricated using photolithography techniques. The
mask (Telic Company, California) for the desired configuration was
made in two steps. The primary mask was exposed in a GCA PG3600F.
Optical Pattern Generator. All the CAD work was done using a VAX
station-3100 Cluster for Computer Aided Design running the physical
layout software SYMBAD (Cadence Design Systems, Inc., California).
After exposure, the mask was developed in developer MF320 (OCG
Microelectronic Materials, Inc., New Jersey). CR-14 Chromium
etchant (CYANTEK Corporation) was used to dissolve the part of the
chromium film not protected with the developed. photoresist. After
this step the photoresist was stripped and the mask was washed in
deionized water and dried in nitrogen. The primary mask for square
production contains a 100.times.100 array of 25 .mu.m squares on 75
.mu.m centers. The secondary mask was made by photo-repeating the
primary mask 30.times.30 times with 5.times. reduction. It was
exposed through the primary mask with the GCA 6300 DSW Projection
Mask Aligner and 5.times. g-line Stepper. The procedure of
photoresist development and chromium etching is the same as for the
primary mask. The number of elements on the secondary mask is
9.times.10.sup.6. Making the secondary mask by repeating the
primary one allowed a decrease in the time required for mask
preparation and reduced the mask cost.
[0037] As substrates, 4-inch silicon p-type wafers were used from
Silicon Quest International Corp., California. The wafers were
washed in acetone and isopropanol and then dried in nitrogen.
Before spin-coating the photoresist the wafer surface was primed by
keeping the wafers in hexamethyldisilazane at 90.degree. C. for 30
minutes. The Yield Engineering Systems LP-III Vacuum Oven was used
for this. Shipley S1813 photoresist was spun at 4000 rpm for 30
sec. plus 3 sec. for acceleration and deceleration. The spun
photoresist was prebaked at 115.degree. C. for 1 minute by placing
the wafer onto a hot plate. The photoresist thickness and
uniformity was checked by the Leitz MV-SP Spectrophotometer. Wafers
were exposed through the secondary mask with UV light using a GCA
6300 DSW Projection Mask Aligner, 5.times. g-line Stepper. There
were 89 prints made on each 4-inch wafer so that the number of the
features on each wafer is 8-10.sup.8.
[0038] Immediately after exposure, the wafers were treated at
90.degree. C. in an ammonia environment (YES oven, 90 min.).
Subsequent flood UV exposure for 2 minutes (Karl Suss MA6 contact
aligner) and development in developer MF32 (OCG Microelectronic
Materials, Inc., New Jersey) was followed by washing in deionized
water and drying in nitrogen; the wafers were ready for metal
deposition. A 200 nm thick layer of copper was thermally deposited
with a CHA thermal evaporator. A 5 nm layer of chromium was
predeposited for better adhesion. By predepositing after forming
the elements, the 5 nm layer of chromium ended up as part of the
particles formed in Examples 1 and 2. The chromium could have been
deposited directly on the silicon wafer and the polymer spun coat
on top of its which would eliminate the chromium from the
particles.
[0039] To lift off the particles the wafer was soaked in acetone
(100 ml) for 2 hours and then sonicated (95HT Tru-Sweep Ultrasonic
Cleaner, Crest Ultrasonic Corp, New Jersey) for 5 seconds. The
suspension was centrifuged (CL International Clinical Centrifuge,
International Equipment CO., Massachusetts) and the acetone was
removed with a pipet. The particles were washed in ethanol 5 times
to remove acetone and photoresist residue.
EXAMPLE 2
[0040] In a manner similar to example 1, Cu/Ni/Cu disks were
fabricated. The diameter of the particles was 1 .mu.m, the
thickness of the metal layers were 100 nm of Cu, 100 nm of Ni and
100 nm of Cu.
EXAMPLE 3
[0041] In a manner similar to example 2 (with no chromium
predeposited), Si/Au/Si disks were made. Silicon and gold layers
were deposited by electron beam evaporation in the CVC SC4500
combination thermal/e-gun evaporation system. The diameter of the
particles was 1 .mu.m, the thickness of the layers were 20 nm of
Si, 150 nm of Au and 20 nm of Si.
EXAMPLE 4
[0042] A mask for W disk fabrication was made in a manner similar
to example 1. Due to the high temperature and the very slow rate of
tungsten deposition in the system used, it was difficult to
thermally deposit thick (>50 nm) layers onto a patterned wafer
without damaging the photoresist. The following technique was used.
A 200 nm layer of W was deposited by sputtering (CVC Sputter
Deposition System) on the top of an Al "sacrificial" layer. 20 nm
of Ta was predeposited onto the Si surface for better adhesion. The
wafers were then primed by exposing them to hexamethyldisilazane in
the YES oven at 90.degree. C. for 30 minutes.
[0043] The photoresist was deposited and developed as before but
the photoresist pattern on the top of the W layer was now used as a
protective mask for etching off some of the surrounding W. To
obtain sharp profiles reactive ion etching in a CF.sub.4 plasma
(RIE System, Applied Materials, California) was used. To lift off
the particles, the Al "sacrificial" layer was dissolved in the
aluminum etchant. Particles were centrifuged and washed in
water.
EXAMPLE 5
[0044] Ellipse-shaped flat W particles (ratio of diagonal axes 2
.mu.m/1 .mu.m) and 200 nm thickness were fabricated.
[0045] The new mask was made in the same way as in the example 4,
except the mask was prepared by 10.times. stepping. The procedures
of metal deposition, wafer washing and particle lift off are
similar to those described in example 4. The primary mask contained
a 100.times.100 array of rhombus with the diagonal axis ratio 50
.mu.m/100 .mu.m with 100 .mu.m spacing between them. The secondary
mask was made by repeating the primary mask 10.times.10 times with
5.times. reduction. It was exposed through the primary mask with
the GCA 630 DSW Projection Mask Aligner, 5.times. g-line Stepper.
The number of elements on the secondary mask is 106. Photoresist
OIR 643 was spun at 4000 rpm for 30 sec. plus 3 sec. for
acceleration and deceleration. The spun-coated photoresist was
prebaked at 90.degree. C. for 1 minute by placing the wafer onto
the hot plate. Wafers were exposed through the secondary mask with
UV light using GCA 6300 DSW Projection Mask Aligner, 10.times.
i-line Stepper. 169 prints were made on each 4-inch wafer so that
the number of the features on the wafer was 1.69.multidot.10.sup.8.
After exposing, the wafers were baked at 115.degree. C. for 1.5
min. on the hot plate. Developer MF4262 (OCG Microelectronic
Materials, Inc., New Jersey) was used to develop the
photoresist.
[0046] Particles made by the inventor in utilizing the processes as
described in the examples are illustrated in FIGS. 6 to 12.
[0047] FIG. 6 illustrates particle 80 formed atop a pillar 82 of
photoresist prior to its separation from the underlying substrate
84.
[0048] FIG. 7 illustrates a plurality of particles 80 secured by
pillars 82 to the underlying substrate 84.
[0049] FIG. 8 illustrates a plurality of uniformly sized and shaped
particles 80 after separation from the pillars 82.
[0050] As shown in FIG. 9, the particles 80 have a definite
disklike shape and are seen to be uniformly sized and shaped.
[0051] As illustrated in FIG. 10, the particles 80 are conveniently
shaped for acceleration and impact into a surface 86 which
illustrates their desirability for biolistic applications.
[0052] FIG. 11 illustrates a magnified view of a particle 80 which
has been propelled into a surface 86. While the particles 80 may
impact a surface 86 on edge so as to lodge therein, as illustrated
in FIGS. 10 and 11, the particles 80 may also "pancake" on the
surface 86 as illustrated in FIG. 12.
[0053] The inventor's description of the preferred embodiment,
including the various alternative processes for producing submicron
particles, and the particles themselves, have been given to
illustrate the various aspects of the invention. One of ordinary
skill in the art would understand that these processes are amenable
for use with various kinds of materials. For example, the material
which comprises the particles themselves would be any materials
amenable to a deposition process. This includes many different
kinds of metals, insulators, semiconductors, ceramics, and glasses,
essentially including any that can be deposited by thermal and
electron-beam evaporation, by electrochemical deposition methods
(electroplating, electroless deposition), by laser ablation and
sputtering of material to be deposited, and any other technique
that allows for material deposition on a surface. The processes
disclosed in the preferred embodiments also utilize a lithographic
process for preparing the substrate. The inventor has used this
lithographic process, i.e. photolithographic process, to actually
prepare a silicon wafer substrate with a layer of photoresist
thereon in making submicron sized particles. However, it should be
understood that the substrate surface may be prepared utilizing any
other suitable process in order to define a pattern on which
particles can be made, and then removed from, the substrate. For
example, with imprint lithography the polymer does not have to be a
photoresist, it can simply be a polymer which imprints well. Or,
the patterned surface may have insulating silicon dioxide regions
which define (through the patterning) conducting regions on a
substrate; certain materials could be electroplated onto these
conducting regions, and then removed (for example by scotch tape
for particles deposited on a relatively non-stick surface).
[0054] In still another aspect of the invention, the formed
particles are separated from the substrate. Various processes are
disclosed herein to achieve that separation. However, any suitable
methodology for separating the particles from the substrate may be
used as the invention should not be viewed as being limited to the
particular methods disclosed, including vibrating, pulling them off
with an adhesive surface such as tape, removal by dissolution of an
underlying sacrificial layer, centrifuging, sonicating, etc. In
still another aspect of the invention, various shapes and sizes of
particles are disclosed. The shapes and sizes of particles which
may be made using the inventor's process are virtually infinite.
Therefore, the invention is not limited to any particular size or
shape, or range of size and shape particle. In still another aspect
of the invention, and as explained above in the examples given and
description of the preferred embodiment, a single harvest of
particles produces a plurality of particles of uniform size and
shape. However, as one of ordinary skill in the art would
understand, the size and shape of the particles is determined by
the preparation of the substrate surface. For example, using the
photolithographic mask disclosed herein, the inventor found it
convenient to utilize techniques which resulted in a single
particle shape and size being chosen and prepared during any
particular pass through the process. One of ordinary skill in the
art would readily understand and appreciate that the invention is
broad enough to encompass the preparation of a photolithographic
mask having a wide range and array of particle shapes and sizes
which may be produced during the same pass of the process. Indeed,
for some applications, it may be desirable to have particles of
varying dimension and shape in the same "harvested" plurality of
particles. Using the inventions disclosed herein, one will be able
to produce this varied collection of particles by predetermining
their shape and size. These advantages and features of the
invention are taught herein and should be considered as part of the
invention.
[0055] The foregoing examples are not meant to be exhaustive and
instead are meant to be illustrative of the scope and content of
the invention. On further thought, one of ordinary skill in the art
would readily understand and appreciate that the teaching of the
specification is broader than that which is contained in the
description of the preferred embodiment and examples given.
Therefore, the scope of the invention should be limited only by the
scope of the claims appended hereto, and their equivalents.
* * * * *