U.S. patent application number 10/418858 was filed with the patent office on 2003-11-20 for methods and apparatus for selective, oxidative patterning of a surface.
Invention is credited to Bearinger, Jane P., Hubbell, Jeffrey A., Michlitsch, Kenneth J..
Application Number | 20030215723 10/418858 |
Document ID | / |
Family ID | 29423574 |
Filed Date | 2003-11-20 |
United States Patent
Application |
20030215723 |
Kind Code |
A1 |
Bearinger, Jane P. ; et
al. |
November 20, 2003 |
Methods and apparatus for selective, oxidative patterning of a
surface
Abstract
The present invention provides methods and apparatus for
selectively patterning surfaces using radical species generated
with a photocatalyst. The photocatalyst may comprise a
photocatalytic semiconductor or a photosensitizer. The radical
species are brought into contact with an oxidizable coating
disposed on the surface, thereby locally oxidizing and selectively
patterning the surface. The photocatalyst is preferably disposed on
a delivery device, such as a stamp, mask, or scanning probe, that
is brought into close proximity or contact with the coated surface.
The photocatalyst is then excited in a manner capable of generating
radical species, for example, oxygen-containing radical species, in
appropriate media. It is expected that these radical species will
be transferred to the coated surface along a substantially shortest
distance path, thereby locally oxidizing and patterning the
surface.
Inventors: |
Bearinger, Jane P.;
(Livermore, CA) ; Hubbell, Jeffrey A.; (Zurich,
CH) ; Michlitsch, Kenneth J.; (Livermore,
CA) |
Correspondence
Address: |
KENNETH J. MICHLITSCH
523 PENNOYER AVE.
GRAND HAVEN
MI
49417
US
|
Family ID: |
29423574 |
Appl. No.: |
10/418858 |
Filed: |
April 18, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60373879 |
Apr 19, 2002 |
|
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Current U.S.
Class: |
430/5 ; 101/401;
249/114.1; 250/310; 378/34; 428/195.1; 430/296; 430/315; 430/320;
430/324; 430/9 |
Current CPC
Class: |
G03F 7/0002 20130101;
B82Y 40/00 20130101; Y10S 430/143 20130101; G03F 7/2014 20130101;
G03F 7/2043 20130101; Y10T 428/24802 20150115; B82Y 10/00
20130101 |
Class at
Publication: |
430/5 ; 430/9;
428/195.1; 250/310; 249/114.1; 101/401; 430/296; 378/34; 430/315;
430/324; 430/320 |
International
Class: |
G03F 001/00; B32B
003/10; G21K 007/00; G01N 023/00; B28B 007/36; B29C 033/56; B41N
001/00; B41B 001/02; G21K 005/00; G03F 007/16; G03F 007/20 |
Claims
What is claimed is:
1. Apparatus for selectively patterning an oxidizable surface, the
apparatus comprising: a photocatalyst; a medium in communication
with the photocatalyst and the oxidizable surface; and an energy
source adapted to excite the photocatalyst, wherein the
photocatalyst generates radical species in the medium upon
excitation by the energy source.
2. The apparatus of claim 1, wherein the oxidizable surface
comprises an oxidizable coating disposed on the surface.
3. The apparatus of claim 1, wherein the photocatalyst comprises a
photocatalytic semiconductor adapted to generate electron hole
pairs upon excitation by the energy source, and wherein the
electron hole pairs generate the radical species in the medium.
4. The apparatus of claim 1, wherein the medium is adapted to
transport the radical species from the photocatalyst to the
oxidizable surface.
5. The apparatus of claim 1, wherein the radical species are
adapted to locally oxidize the the oxidizable surface at points
where the radical species contact the surface.
6. The apparatus of claim 5, wherein locally oxidizing the
oxidizable surface comprises locally pattering the surface.
7. The apparatus of claim 3, wherein excitation of the
photocatalytic semiconductor by the energy source comprises
excitation above a band gap of the photocatalytic
semiconductor.
8. The apparatus of claim 1, wherein the photocatalyst is chosen
from the group consisting of photocatalytic semiconductors,
TiO.sub.2, SnO.sub.2, compounds of InTaO.sub.4 doped with Ni,
photosensitizers, photofrins, texaphyrins, metallotexaphyrins,
porphyrins, hematoporphyrins, chlorins, bacteriochlorins,
phthalocyanines, purpurins, and combinations thereof.
9. The apparatus of claim 1 further comprising a delivery device on
which the photocatalyst is disposed.
10. The apparatus of claim 9, wherein the delivery device is chosen
from the group consisting of stamps, masks, probes, scanning
probes, and combinations thereof.
11. The apparatus of claim 9, wherein the photocatalyst is disposed
on the delivery device in a specified pattern.
12. The apparatus of claim 11, wherein the specified pattern
comprises a pattern chosen from the group consisting of an entirety
of the delivery device, a 2-dimensionally patterned section of the
delivery device, a 3-dimensionally patterned section of the
delivery device, a tip of a scanning probe, a localized region of
the delivery device, and combinations thereof.
13. The apparatus of claim 11, wherein the specified pattern is
fabricated using e-beam lithography.
14. The apparatus of claim 1, wherein the energy source is chosen
from the group consisting of visible light sources, UV sources,
x-ray sources, visible light lamps, UV lamps, x-ray lamps, mercury
lamps, visible light lasers, HeNe lasers, UV lasers, x-ray lasers,
pulsed lamps, pulsed lasers, and combinations thereof.
15. The apparatus of claim 1, wherein the oxidizable surface
comprises a surface chosen from the group consisting of alkane
thiols, thioethers, unsaturated materials, saturated materials,
bare metal surfaces, metal oxides, and combinations thereof.
16. The apparatus of claim 9 further comprising a second
photocatalyst disposed on the delivery device.
17. The apparatus of claim 11, wherein the delivery device inhibits
transmission of excitation energy provided by the energy source
outside of the specified pattern.
18. The apparatus of claim 1, wherein the medium comprises a fluid
medium.
19. The apparatus of claim 1, wherein the medium comprises an
oxidant chosen from the group consisting of oxygen, nitrogen,
oxidizing ions, Redox species, Redox mediators, electron transfer
agents, and combinations thereof.
20. The apparatus of claim 1, wherein the medium comprises a medium
chosen from the group consisting of gaseous mediums, liquid
mediums, aqueous mediums, organic mediums, inorganic mediums,
water, gels, air, oxygen-containing mediums, nitrogen-containing
mediums, thiohexamic ester-containing mediums, Argon gas, vacuum,
and combinations thereof.
21. The apparatus of claim 1 further comprising a stabilizing
agent.
22. The apparatus of claim 21, wherein the stabilizing agent is
chosen from the group consisting of selenium, zinc, lipoic acid,
methionine, cysteine, N,N Dimethyl glycine, and combinations
thereof.
23. A method for selectively patterning an oxidizable surface, the
method comprising: providing a photocatalyst in close proximity or
contact to the surface; exciting the photocatalyst; generating
radical species with the excited photocatalyst; transferring the
radical species to the surface; and locally oxidizing the surface
at points where the radical species contact the surface, thereby
selectively patterning the surface.
24. The method of claim 23, wherein the photocatalyst comprises a
photocatalytic semiconductor, and wherein exciting the
photocatalyst comprises forming electron hole pairs in or on the
photocatalytic semiconductor.
25. The method of claim 24, wherein forming electron hole pairs in
or on the photocatalytic semiconductor comprises exciting the
photocatalytic semiconductor above its band gap.
26. The method of claim 24, wherein generating radical species with
the excited photocatalyst comprises generating radical species with
the electron hole pairs.
27. The method of claim 26, wherein generating radical species with
the electron hole pairs comprises generating species by contacting
the electron hole pairs with a medium in communication with the
photocatalytic semiconductor.
28. The method of claim 23, wherein the photocatalyst comprises a
photosensitizer.
29. The method of claim 23, wherein transferring the radical
species to the surface comprises transferring the radical species
to the surface through a medium.
30. The method of claim 23, wherein the medium comprises a fluid
having an oxidant.
31. The method of claim 23, wherein providing a photocatalyst
comprises providing a photocatalyst chosen from the group
consisting of photocatalytic semiconductors, TiO.sub.2, SnO.sub.2,
compounds of InTaO.sub.4 doped with Ni, photosensitizers,
photofrins, texaphyrins, metallotexaphyrins, porphyrins,
hematoporphyrins, chlorins, bacteriochlorins, phthalocyanines,
purpurins, and combinations thereof.
32. The method of claim 23, wherein providing a photocatalyst
comprises providing a photocatalyst disposed on a delivery
device.
33. The method of claim 32, wherein providing a photocatalyst
disposed on a delivery device comprises providing a photocatalyst
disposed on a delivery device chosen from the group consisting of
stamps, masks, probes, scanning probes, and combinations
thereof.
34. The method of claim 23, wherein locally oxidizing the surface
comprises locally oxidizing a surface chosen from the group
consisting of alkane thiols, thioethers, unsaturated materials,
saturated materials, bare metal surfaces, metal oxides, and
combinations thereof.
35. The method of claim 23 further comprising: providing a second
photocatalyst in close proximity or contact to the surface;
exciting the second photocatalyst; generating a second set of
radical species with the second excited photocatalyst; transferring
the second set of radical species to the surface; and locally
oxidizing the surface at points where the second set of radical
species contact the surface, thereby selectively patterning the
surface with a second pattern.
36. The method of claim 35, wherein the photocatalyst and the
second photocatalyst comprise different photocatalysts.
37. The method of claim 23, wherein selectively patterning the
surface comprises selectively patterning the surface with a pattern
chosen from the group consisting of positive patterns, negative
patterns, continuous patterns, discontinuous patterns, multi-step
patterns, one-dimensional patterns, two-dimensional patterns,
three-dimensional patterns, and combinations thereof.
38. The apparatus of claim 1, wherein a bias voltage is applied to
the oxidizable surface.
39. A patterned surface made by a process comprising: generating
radical species with a photocatalyst; and locally oxidizing the
surface with the radical species to pattern the surface.
40. The patterned surface made by the process of claim 39, wherein
generating radical species with the photocatalyst further comprises
generating radical species with a photocatalytic semiconductor.
41. The patterned surface made by the process of claim 40, wherein
generating radical species with the photocatalytic semiconductor
further comprises forming electron hole pairs in the photocatalytic
semiconductor that form radical species upon exposure to
appropriate media.
42. The patterned surface made by the process of claim 39, wherein
generating radical species with the photocatalyst further comprises
generating radical species with a photosensitizer.
43. The patterned surface made by the process of claim 39, wherein
generating radical species with the photocatalyst further comprises
exciting the photocatalyst with an energy source.
44. The patterned surface made by the process of claim 39, wherein
locally oxidizing the surface with the radical species to pattern
the surface comprises patterning the surface with features having a
size smaller than about 100 nm.
45. The patterned surface made by the process of claim 39, wherein
locally oxidizing the surface with the radical species to pattern
the surface comprises patterning the surface with a resolution
finer than about 100 nm.
46. The patterned surface made by the process of claim 39, wherein
generating radical species with the photocatalyst further comprises
generating radical species with a photocatalyst disposed on a
delivery device.
47. The patterned surface made by the process of claim 46, wherein
generating radical species with a photocatalyst disposed on a
delivery device further comprises generating radical species with a
photocatalyst disposed on a delivery device chosen from the group
consisting of stamps, masks, probes, scanning probes, and
combinations thereof.
48. The patterned surface made by the process of claim 46, wherein
generating radical species with a photocatalyst disposed on a
delivery device further comprises generating radical species with a
photocatalyst disposed on a delivery device in a specified
pattern.
49. The patterned surface made by the process of claim 48, wherein
generating radical species with a photocatalyst disposed on a
delivery device in a specified pattern further comprises generating
radical species with a photocatalyst disposed on a delivery device
in a specified pattern that has features with a size smaller than
100 nm.
50. The patterned surface made by the process of claim 48, wherein
locally oxidizing the surface with the radical species to pattern
the surface comprises transferring the radical species from the
specified pattern on the delivery device to the surface along a
substantially shortest distance path.
51. The patterned surface made by the process of claim 39, wherein
generating radical species with a photocatalyst further comprises
generating radical species with a photocatalyst chosen from the
group consisting of photocatalytic semiconductors, TiO.sub.2,
SnO.sub.2, compounds of InTaO.sub.4 doped with Ni,
photosensitizers, photofrins, texaphyrins, metallotexaphyrins,
porphyrins, hematoporphyrins, chlorins, bacteriochlorins,
phthalocyanines, purpurins, and combinations thereof.
Description
REFERENCE TO RELATED APPLICATION
[0001] The present application claims priority and the benefit of
the filing date of provisional U.S. patent application Ser. No.
60/373,879 filed Apr. 19, 2002, and takes advantage of that filing
date.
FIELD OF THE INVENTION
[0002] The present invention is related to surface patterning. More
particularly, this invention is related to methods and apparatus
for selectively patterning a surface using radical species, thereby
providing a surface with a specified and controllable gradient of
electrical, chemical, and/or physical properties.
BACKGROUND OF THE INVENTION
[0003] Electron beam ("e-beam") lithography has successfully been
employed in a variety of industrial applications to fabricate very
small structures. An e-beam is focused on a target substrate to
slowly and painstakingly `draw`, `carve`, or ablate a very fine
pattern into the substrate. This procedure is repeated for each
substrate required. E-beam lithography typically is capable of
producing features having a dimension or resolution on the order of
nanometers.
[0004] Though often effective, e-beam lithography is prohibitively
slow and expensive for many applications, and is not readily
applicable to mass-production. Techniques therefore have been
developed to lower costs, decrease production times, and increase
reproducibility. One such technique comprises using e-beam
lithography to create a master, from which a stamp may be
secondarily created. A stamping material (ink) is applied to the
stamp, which is subsequently brought into contact with a surface.
The stamping material is transferred to the surface at locations
where the stamp contacts the surface. The surface may then be
etched to remove surface material at all points that do not have
stamping material, thereby replicating the stamp and selectively
patterning the surface. Stamping of alkane thiols typically is
capable of producing features having a dimension or resolution on
the order of microns, though smaller structures are theoretically
attainable.
[0005] Stamping of alkane thiols from a stamp onto a gold surface
has been extensively investigated. The alkane thiol is absorbed
either into or onto the stamp, and is then brought into contact
with the gold substrate surface. Alkane thiols commonly consist of
close-packed, independent chains that may be chemisorbed to a
surface, and which often are used to modify surfaces, for example,
to alter corrosion resistance and/or electrical properties, or to
pattern the surfaces. Common alkane thiols include octadecanethiol
and hexadecanethiol. These materials are typically applied from
solution, e.g. ethanol or hexane, to surfaces such as gold, silver,
or copper.
[0006] Although stamping of alkane thiols on gold surfaces has been
extensively investigated, to date the method is still primarily a
laboratory technique that has not been effectively transferred to
industrial settings, due to the complexities of the stamping
process. The simultaneous and often contradictory requirements of
rapid diffusion and high solubility of the alkane thiol onto the
stamp, appropriate mechanical characteristics of the stamp, fast
reaction rates relative to surface diffusion rates of the alkane
thiol onto the gold substrate, high irreversibility on the gold
surface, and resistance of the stamping material to subsequent
processing steps have been difficult to achieve. Thus, a central
factor limiting adaptation of the laboratory technique to
industrial applications has been the difficulties encountered while
trying to achieve simultaneous control of multiple time-dependent,
or rate, processes.
[0007] A newer surface patterning technique that has been developed
to lower costs and decrease production times associated with e-beam
lithography employs e-beam, V, or x-ray resists. Such resists, and
techniques for manufacturing them, are found, for example, in U.S.
Pat. Nos. 4,717,645 to Kato et al.; 4,795,692 to Anderson et al.;
and 4,868,241 to Hiscock et al.; all of which are incorporated
herein by reference. A common resist technique comprises coating a
substrate with a material that is sensitive to e-beam, UV, or x-ray
radiation. The coating is selectively exposed to radiation, for
example, with a focused electron beam that `traces` the required
pattern on the coating. Irradiation removes the coating at the
point of exposure and provides a selectively patterned surface.
This technique is similar to traditional e-beam lithography, except
that the affected material comprises only a very thin, typically
organic coating, thereby reducing the amount of material that is
removed and the amount of time required to achieve patterning. The
size of features attainable using resists depends on the energy
source used for irradiation.
[0008] A significant drawback of resist techniques is that,
although more rapid than traditional e-beam lithography techniques,
time- and cost-intensive patterned irradiation of resists must
still be conducted individually for each patterned surface. This
drawback significantly limits the industrial viability of e-beam
and x-ray resists.
[0009] Yet another technique that reduces the costs and production
times associated with e-beam lithography is photolithography.
Photolithography was developed prior to e-beam techniques, but
provides many of the benefits of stamping and resist techniques.
Photolithography typically requires production of a Master mask.
The mask is placed over a substrate that has been coated with a
photosensitive resist. A light source is shone through the
patterned mask onto the resist, thereby patterning the surface.
With a positive resist, material may be easily removed at all
points on the surface that are exposed to irradiation. With a
negative resist, material may be removed at all points not
irradiated.
[0010] Although photolithography provides many of the benefits of
e-beam lithography in a rapid and low cost procedure, the technique
has fundamental limits. Specifically, photolithography typically
cannot pattern surface structures having a size much smaller than
the wavelength of the incident light. When using an i-line standard
(365 nm UV light generated with mercury lamps) energy source,
features on the order of about 500 nm are possible. Advanced
focusing techniques may allow features slightly smaller than the
wavelength of the incident light, for example, features as small as
300 nm with the i-line standard, but significantly smaller features
are not possible.
[0011] Researchers have also examined the possibility of patterning
with deep UV ("DUV") light having a wavelength of 248 nm, generated
with a krypton fluoride ("KrF") excimer laser energy source 18.
Furthermore, researchers have explored 193 nm laser sources 18,
such as argon fluoride ("ArF") excimer lasers. Researchers are
still further exploring 157 nm laser sources 18, in the hopes of
patterning surface features on the order of about 100 nm, when
using advanced focusing techniques. However, systems using focusing
techniques and operating at or below about 193 nm may suffer from
degraded optics, since most lens materials, including fused silica
or quartz, are absorptive at these wavelengths. Density variations
in materials are also a problem at or below about 193 nm. Exotic
alternative lens materials therefore are being examined, including,
for example, calcium fluoride. Although calcium fluoride is highly
transmissive, a significant drawback is that it is very difficult
to fabricate. Additionally, if extreme UV (13 nm) or X-ray (<3
nm) are light sources ever considered for mass-production purposes,
such as in the production of microelectronics, it is expected that
complex and cost-intensive new lasers or synchrotron systems will
be required to generate adequate extreme UV or X-ray photons to
meet production requirements.
[0012] Especially in the field of microelectronics, the drive for
smaller and smaller structures is rapidly creating a need to
pattern surface structures smaller than those possible today with
standard photolithography employing i-line standard UV light. In
many cases, traditional e-beam techniques are the only practical
recourse for providing such fine structures.
[0013] In view of the drawbacks associated with prior art
patterning techniques, it would be desirable to provide methods and
apparatus for patterning surfaces that overcome these
drawbacks.
[0014] It would be desirable to provide methods and apparatus that
reduce costs and production times, as compared to e-beam
techniques.
[0015] It also would be desirable to provide methods and apparatus
for patterning surfaces that require control of fewer rate
processes.
[0016] It would be desirable to provide methods and apparatus for
patterning surfaces that may be replicated using a stamping or
masking technique.
[0017] It would be desirable to provide methods and apparatus that
theoretically enable patterning of surface structures having a size
smaller than achievable with standard photolithography
techniques.
[0018] It would be desirable to provide methods and apparatus that
are applicable to industrial applications.
SUMMARY OF THE INVENTION
[0019] In view of the foregoing, it is an object of the present
invention to provide methods and apparatus for patterning surfaces
that overcome drawbacks associated with prior art techniques.
[0020] It is an object to provide methods and apparatus that reduce
costs and production times, as compared to e-beam techniques.
[0021] It is another object of the present invention to provide
methods and apparatus that require control of fewer rate
processes.
[0022] It is yet another object to provide methods and apparatus
for patterning surfaces that may be replicated using a stamping or
masking technique.
[0023] It is still another object to provide methods and apparatus
that theoretically enable patterning of surface structures having a
size smaller than achievable with standard photolithography
techniques.
[0024] It is an object to provide methods and apparatus that are
applicable to industrial applications.
[0025] These and other objects of the present invention are
accomplished by patterning a surface using radical species
generated with a photocatalyst, for example, a photocatalytic
semiconductor, a photosensitizer, or a combination thereof. The
radical species are selectively brought into contact with an
oxidizable coating disposed on the surface.
[0026] In a preferred embodiment, the oxidizable surface coating is
adsorbed onto the surface. The coated surface is preferably
immersed in a medium capable of generating radical species in the
presence of electron hole pairs or excited molecules, for example,
an oxygen- or nitrogen-containing medium. The medium may be either
organic or inorganic and is preferably fluidic, for example, a
gaseous medium, a liquid medium, an aqueous medium, a gel, water,
or air. Furthermore, the medium preferably comprises an oxidant,
such as oxygen, nitrogen, oxidizing ions, Redox species, Redox
mediators, or electron transfer agents. The medium may also or
alternatively contain stabilizing agents, such as selenium, zinc,
lipoic acid, methionine, cysteine, or N,N Dimethyl glycine. As yet
another alternative, the medium may comprise more inert conditions,
such as vacuum or Argon gas. Other mediums will be apparent to
those of skill in the art.
[0027] A stamp or mask, formed, for example, using traditional
e-beam lithography techniques, per se known, is brought into close
proximity or contact with the coated surface. The mask comprises a
patterned layer of material that is capable of generating radical
species, for example, a patterned photocatalyst layer. When the
photocatalyst comprises a photocatalytic semiconductor, TiO.sub.2
is a preferred photocatalytic semiconductor, but others, such as
SnO.sub.2, or an InTaO.sub.4 compound doped with Ni, will be
apparent to those of skill in the art and are included in the scope
of the present invention. When the photocatalyst comprises a
photosensitizer or photosensitizing agent, preferred
photosensitizers include photofrins, texaphyrins,
metallotexaphyrins, porphyrins, hematoporphyrins, chlorins,
bacteriochlorins, phthalocyanines and purpurins. Additional
photosensitizers will be apparent to those skilled in the art and
are included in the present invention.
[0028] Next, an energy source is exposed through the mask/stamp to
the patterned photocatalyst layer. It is expected that the
photocatalyst will generate radical species in appropriate
environments upon exposure to the energy source. When the
photocatalyst comprises a photocatalytic semiconductor, preferred
light sources include UV or x-ray lamps or lasers. Other light
sources will be apparent to those skilled in the art. Energy from
the light source generates electron hole pairs in/on the patterned
photocatalytic semiconductor layer, for example, in a patterned
layer of TiO.sub.2. The electron hole pairs generate radical
species, such as oxygen-containing radical species, in appropriate
environments.
[0029] When the photocatalyst comprises a photosensitizer,
preferred light sources include visible light sources, such as
lights sources with wavelengths between about 550-850 nm, for
example, a visible laser light source, such as a Helium Neon
("HeNe") laser. Other light sources, such as UV light sources, will
be apparent. Energy from the light source excites the
photosensitizer from a ground state to a singlet excited state. The
singlet may decay to an intermediate triplet excited state, which
is able to transfer energy to another triplet. Some molecules have
a triplet ground state, for example, oxygen or O.sub.2. Thus,
energy may be transferred from the photosensitizer in the excited
triplet state to the triplet ground state molecule, thereby
exciting the molecule to a singlet state. A radical-generating
reaction may then be achieved with the excited singlet state
molecule, for example, a reaction generating oxygen-containing
radical species. Other molecules capable of forming radical species
upon exposure to an excited photosensitizer will be apparent to
those of skill in the art, for example, thiohydroxamic esters.
[0030] Regardless of whether the patterned photocatalyst layer
comprises a patterned photocatalytic semiconductor layer or a
patterned photosensitizer layer, it is expected that radical
species generated at the patterned photocatalyst layer will be
transferred to the coated surface along a substantially shortest
distance path. Thus, only areas on the coated surface that are in
close proximity to the patterned layer of the mask/stamp will come
into contact with the radical species. Since the surface coating is
oxidizable, it is expected that these areas will oxidize locally,
thereby patterning the surface. Portions that are not contacted by
the radical species are not expected to oxidize. It should also be
noted that oxidation may be possible with excited singlet or
triplet state molecules, in addition to radical species.
[0031] Techniques of the present invention potentially may be used
in combination with prior art photosensitive resists. Such local
patterning through chemical modification of the coating is expected
to alter the reactivity of the coating, and may either stabilize or
destabilize the affected portion of the coated surface. Unaffected
adsorbed material optionally may be used for a second chemical
step, for example, a second masking step.
[0032] An expected advantage of the present invention, as compared
to prior art photolithography techniques, is that the patterned
mask/stamp's photocatalyst layer will enable patterning of features
on the coated surface that are significantly smaller than the
wavelength of light generated by the energy source. When using a
photocatalytic semiconductor, this is possible because electron
hole pairs generated in the photocatalytic semiconductor layer have
a dimension on the order of sub-Angstroms, as compared to the
incident light that generates the electron hole pairs, which has a
dimension on the order of nanometers. Likewise, when using
photosensitizers, the radical species generated with the
photosensitizers by quanta of energy transmitted to molecules, are
expected to be significantly smaller than the wavelength of
incident light.
[0033] An alternative embodiment of apparatus in accordance with
the present invention comprises a scanning probe having a
photocatalyst tip. An energy source is coupled to the tip, for
example, via fiber optics or near-field optical microscopy, such
that radical species may be generated locally at the tip. By
scanning the probe over an oxidizable surface coating while
creating radical species, a selectively patterned surface may be
formed.
[0034] It is expected that the present invention may be used in
conjunction with a variety of oxidizable surface coatings. In a
first embodiment, the surface coatings comprise alkane thiols. In a
second embodiment, the coatings comprise thioethers. In a third
embodiment, the coatings comprise unsaturated materials. Saturated
materials are also contemplated. In a fourth embodiment, the
coatings comprise metal oxides. Bare metal substrates may also be
used. Other coatings will be apparent to those skilled in the
art.
[0035] The present invention may be applicable to a variety of
fields ranging from fabrication of microelectronics, computer
chips, biomedical assays, physical research (e.g. top gates and
quantum dots or wells), and combinatorial chemistry. Additional
applications will be apparent to those of skill in the art, and are
included in the present invention.
[0036] Methods and apparatus for accomplishing the present
invention are provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] Further features of the invention, its nature and various
advantages will be more apparent from the accompanying drawings and
the following detailed description of the preferred embodiments, in
which like reference numerals refer to like parts throughout, and
in which:
[0038] FIG. 1 is a schematic representation of a prior art
technique for performing photolithography;
[0039] FIGS. 2A-2C are schematic representations of photocatalyst
reactions leading to generation of radical species; FIGS. 2A and 2B
depict the formation of electron hole pairs in a photocatalytic
semiconductor, while FIG. 2C depicts excitation of a
photosensitizer;
[0040] FIGS. 3A-3D are schematic representations of chemical
reactions demonstrating oxidation of a surface coating in the
presence of radical species;
[0041] FIGS. 4A-4C are schematic representations of a first
embodiment of apparatus constructed in accordance with the present
invention;
[0042] FIGS. 5A and 5B are schematic representations of a method of
patterning a surface in accordance with the present invention,
utilizing the apparatus of FIG. 4;
[0043] FIGS. 6A and 6B are a schematic representation of an
alternative embodiment of apparatus constructed in accordance with
the present invention;
[0044] FIGS. 7A and 7B are schematic representations of a method of
patterning a surface in accordance with the present invention,
utilizing the apparatus of FIG. 6A;
[0045] FIG. 8 is a schematic representation of yet another
alternative embodiment of apparatus constructed in accordance with
the present invention;
[0046] FIGS. 9A-9C are schematic representations of a method of
patterning a surface in accordance with the present invention,
utilizing the apparatus of FIG. 8; and
[0047] FIGS. 10A-10E are schematic representations of exemplary
surface patterns that it is expected may be formed utilizing the
methods and apparatus of the present invention; FIG. 10 are
overhead views, except for FIG. 10C, which is a side view.
DETAILED DESCRIPTION OF THE INVENTION
[0048] The present invention is related to surface patterning. More
particularly, this invention is related to methods and apparatus
for selectively patterning a surface using radical species, thereby
providing a surface with a specified and controllable gradient of
electrical, chemical, and/or physical properties.
[0049] With reference to FIG. 1, a prior art technique for
performing photolithography is described. Substrate 10 comprises
surface 12 having oxide 14 and photosensitive resist coating 15.
Mask 16, having transparent pattern section 17 and opaque masking
section 18, is disposed above oxide 14 and coating 15, while energy
source 19 is disposed above mask 16. Energy source 19 typically
comprises a UV or x-ray energy source.
[0050] A common technique for forming substrate 10 with surface 12,
oxide 14, and photosensitive resist coating 15 comprises providing
a doped silicon substrate 10. Oxide 14 is then grown on substrate
10. Next photoresist 15 is spun-coated onto the oxide.
[0051] With mask 16 disposed between surface 12 and energy source
18, the energy source is activated and irradiates mask 16 with
incident light 20. Incident light 20 passes through mask 16 along
pattern section 17, and contacts photosensitive resist coating 15
in a pattern 17'. Pattern 17' replicates pattern section 17 of mask
16 on surface 12. Masking section 18 inhibits transmission of light
20 to surface 12.
[0052] Resist coating 15 may be either a positive or a negative
resist coating. With positive resist coating PR, coating 15 may be
easily removed at all points on surface 12 disposed within pattern
17' that are exposed to irradiation, for example, via a developing
procedure. With negative resist NR, material may be removed at all
points on surface 12 that are not disposed within pattern 17',
again via a developing procedure. Oxide 14 may then be removed at
all points where photosensitive resist coating 15 has been removed,
for example, via a secondary etching procedure. Selective removal
of oxide 14 provides selectively patterned surfaces 12' and 12",
respectively.
[0053] Although photolithography provides many of the benefits of
e-beam lithography in a rapid and low cost procedure, the technique
has fundamental limits. Specifically, photolithography typically
cannot pattern surface structures having a size much smaller than
the wavelength of the incident light from energy source 18. This
means that the minimum size of structures contained within pattern
17 of mask 16 must be close to the dimensions of the wavelength of
the incident light, and the resultant selective pattern 17' formed
on surface 12 will not have any structures significantly smaller or
finer than the structures within mask pattern 17.
[0054] When using an i-line standard (365 nm UV light generated
with mercury lamps) energy source 18, features on the order of
about 500 nm are possible. Advanced focusing techniques may allow
features or structures slightly smaller than the wavelength of the
incident light, for example, features as small as 300 nm, but
features significantly smaller than the wavelength of the incident
light have not been achieved. Structures on the order of 300-500 nm
may not be sufficient in a variety of applications, including
microelectronics. Thus, expensive and time-consuming e-beam
techniques may be required.
[0055] Researchers have also examined- the possibility of
patterning with deep UV ("DUV") light having a wavelength of 248
nm, generated with a krypton fluoride ("KrF") excimer laser energy
source 18. Furthermore, researchers have explored 193 nm laser
sources 18, such as argon fluoride ("ArF") excimer lasers.
Researchers are still further exploring 157 nm laser sources 18, in
the hopes of patterning surface features on the order of about 100
nm, when using advanced focusing techniques. However, systems using
focusing techniques and operating at or below about 193 nm may
suffer from degraded optics, since most lens materials, including
fused silica or quartz, are absorptive at these wavelengths.
Density variations in materials are also a problem at or below
about 193 nm. Exotic alternative lens materials therefore are being
examined, including, for example, calcium fluoride. Although
calcium fluoride is highly transmissive, a significant drawback is
that it is very difficult to fabricate. Additionally, if extreme UV
(13 nm) or X-ray (<3 nm) are light sources ever considered for
mass-production purposes, such as in the production of
microelectronics, it is expected that complex and cost-intensive
new lasers or synchrotron systems will be required to generate
adequate extreme UV or X-ray photons to meet production
requirements.
[0056] Referring now to FIGS. 2 and 3, prior to discussion of
apparatus and methods in accordance with the present invention,
reactions encountered while practicing the present invention are
described. Although these reactions are believed to be the
mechanism by which the present invention may be practiced, the
present invention is primarily concerned with the end result, i.e.
patterning. Thus, the reactions and purported mechanism are
provided only for the benefit of the reader and should in no way be
construed as limiting.
[0057] With reference to FIG. 2, photocatalyst reactions leading to
generation of radical species are described. FIGS. 2A and 2B depict
the formation of an electron hole pair in a photocatalytic
semiconductor atom, with subsequent generation of radical species.
FIG. 2C depicts excitation of a photosensitizer.
[0058] In FIG. 2A, photocatalytic semiconductor atom S is disposed
in an oxygen-containing medium M, for example, H.sub.2O ).
Semiconductor atom S is contacted by energy quanta E.sub.1 having
an excitation energy below the band gap energy of semiconductor
atom S. As an illustrative example, the band gap energy for
photocatalytic semiconductor TiO.sub.2 is about 3.2 eV. Since
energy quanta E.sub.1 has an excitation energy below the band gap
of semiconductor atom S, the quanta does not generate an electron
hole pair in semiconductor atom S.
[0059] In FIG. 2B, semiconductor atom S is contacted by energy
quanta E.sub.2 having an excitation energy above the band gap of
semiconductor atom S. Energy quanta E.sub.2 releases electron e and
hole h within semiconductor S, which are collectively referred to
as electron hole pair H. Electron hole pair H migrates to
atom/medium interface I. Electron e and hole h interact with oxygen
contained within medium M, thereby forming oxygen-containing
radical species R.sub.1 and R.sub.2. R.sub.1 is a hydroxyl radical,
while R.sub.2 is a super-anion oxide radical. Radical species
R.sub.1 and R.sub.2 have cross-sections on the order of Angstroms
or smaller. After a brief period, electron hole pairs that don't
form radical species recombine.
[0060] For the exemplary embodiment of a TiO.sub.2 photocatalytic
semiconductor atom S exposed to energy quanta E.sub.2 from a UV
energy source, while immersed in fluid medium M comprising
H.sub.2O, the equations governing generation of radical species are
as follows:
TiO.sub.2+UV.fwdarw.e+h (1)
h+OH--.fwdarw.*OH (2)
e+O.sub.2.fwdarw.O.sub.2*-- (3)
O.sub.2*--+H.sub.2O .fwdarw.HO.sub.2*+OH-- (4)
[0061] where `*` denotes a radical species. This provides an
overall reaction via TiO.sub.2 catalysis of:
UV+O.sub.2+H.sub.2O.fwdarw.HO.sub.2*+*OH (5)
[0062] Although FIGS. 2A and 2B are described with respect to an
oxygen-containing medium, other mediums containing other elements
capable of generating radical species in the presence of electron
hole pairs will be apparent to those of skill in the art. One such
medium is a nitrogen-containing medium. Others include reagents
that may react across an unsaturated bond via a Michael-type
addition mechanism.
[0063] Referring now to FIG. 2C, photosensitizer Ph is excited from
ground state P.sup.0 to excited singlet state .sup.1p* by energy
quanta E.sub.3. Photosensitizer Ph decays from singlet state
.sup.1p* to intermediate excited triplet state .sup.3p* While
disposed in the triplet state, photosensitizer Ph is able to
transfer energy to another triplet state molecule. Some molecules
have a triplet ground state, for example, oxygen O.sub.2, which is
used in the exemplary embodiment of FIG. 2C.
[0064] As seen in FIG. 2C, energy is transferred from excited
triplet state photosensitizer .sup.3p* Ph to triplet ground state
oxygen molecule .sup.3O.sub.2, thereby exciting the .sup.3O.sub.2
molecule to an excited singlet state .sup.1O.sub.2. A
radical-generating reaction may then be achieved with the excited
singlet state molecule .sup.1O.sub.2, for example, a reaction that
generates oxygen-containing radical species. Other molecules
capable of forming radical species upon exposure to an excited
photosensitizer will be apparent to those of skill in the art, for
example, thiohydroxamic esters.
[0065] With reference to FIG. 3, oxidation of a surface coating in
the presence of radical species is described. It should also be
noted that oxidation may be possible with excited singlet or
triplet state molecules, in addition to radical species. In FIG.
3A, oxidizable surface coating C, disposed on substrate Su, is
contacted by radical species R. Radical species R causes surface
coating C to locally oxidize where the radical species contacts the
surface coating at point P, as seen in FIG. 3B. The cross-section
of point P may be on the order of angstroms or smaller.
[0066] The chemistry of coating C may be chosen such that the
reactivity of the coating may be altered at point P, and may either
stabilize or destabilize point P of coating C. For example,
dependent on the chemistry of coating C and/or secondary processing
techniques, point P of coating C may be removed from coating C, as
seen in FIG. 3C. Alternatively, coating C may be removed from
substrate Su at all positions except point P, as seen in FIG.
3D.
[0067] With reference now to FIG. 4, a first embodiment of
apparatus in accordance with the present invention is described.
Apparatus 30 comprises substrate 32 having surface 34 with
oxidizable coating 36. Apparatus 30 further comprises mask 40
having mask section 42 and pattern section 44. Photocatalyst layer
46 is disposed beneath mask section 42 and pattern section 44.
Apparatus 30 also comprises energy source 50. Apparatus 30 still
further comprises medium M in which oxidizable coating 36 and
photocatalyst layer 46 are immersed. Mask 40 is disposed between
substrate 32 and energy source 50.
[0068] Mask section 42 of mask 40 preferably comprises a shielding
material, for example, a UV or x-ray absorber or quencher, carbon,
or a metal such as lead, or gold, which is capable of inhibiting
transmission of energy irradiated by energy source 50. Mask section
42 may also comprise a material capable of quenching radical
species, such as selenium or zinc. Additional materials for mask
section 42 will be apparent to those of skill in the art.
[0069] Pattern section 44 comprises the portion of mask 40 defining
the pattern to be replicated on surface 34 of substrate 32. In FIG.
4A, pattern section 44 comprises either a material capable of
transmitting energy provided by energy source 50, or voids formed
within mask section 42, for example, drilled within mask section 42
to expose photocatalytic semiconductor layer 46. Pattern section 44
further comprises the portions of layer 46 disposed beneath such
voids or transmitting material.
[0070] Photocatalyst layer 46 may comprise a photocatalytic
semiconductor layer, a photosensitizer layer, or a combination
thereof. For the purposes of the present invention, a photocatalyst
is defined as a material that is capable of producing a
photochemical and/or photophysical alteration in a system, without
being consumed by the alteration. When the photocatalyst comprises
a photocatalytic semiconductor, TiO.sub.2 is a preferred
photocatalytic semiconductor, but others, such as SnO.sub.2, or an
InTaO.sub.4 compound doped with Ni, will be apparent to those of
skill in the art and are included in the scope of the present
invention. When the photocatalyst comprises a photosensitizer or
photosensitizing agent, preferred photosensitizers include
photofrins, texaphyrins, metallotexaphyrins, porphyrins,
hematoporphyrins, chlorins, bacteriochlorins, phthalocyanines and
purpurins. Additional photosensitizers will be apparent to those
skilled in the art and are included in the present invention.
[0071] It is also contemplated that substrate 47 may be attached to
mask 40, as seen in FIG. 4B. Substrate 47 may be attached either to
layer 46, as in FIG. 4B, or to the shielding material of mask
section 42. With substrate 47 attached to layer 46, mask 40 is
preferably positioned such that mask section 42, as well as the
transmitting portion of pattern section 44, is disposed closest to
coating 36, while layer 46 is disposed between the shielding layer
and the substrate. A preferred substrate comprises fused silica or
quartz, however other substrates will be apparent.
[0072] A variety of materials and techniques may be used to form
mask 40 having mask section 42 and pattern section 44. In a first
embodiment, mask 40 is formed as a bilayer material. The first
layer comprises a shielding material, as described above with
respect to mask section 42. The second layer comprises the
photocatalyst layer 46, also described previously. Optionally,
substrate 47 may be included as a third layer. A portion of the
shielding layer is then selectively removed, for example, using
e-beam or traditional machining techniques, to expose layer 46 and
form pattern section 44, as well as mask section 42.
[0073] In a second embodiment, mask 40 is formed of
Poly(dimethylsiloxane) ("PDMS"). In this embodiment, PDMS mask 40
may be dipped in a solution of the photocatalyst just after curing.
Alternatively, the photocatalyst may be painted, flame-coated, or
vapor deposited on the surface. Portions exposed to the
photocatalyst comprise pattern section 44, while other portions
comprise mask section 42.
[0074] In a third embodiment, mask 40 comprises polymers, such as
Polyvinyl chloride ("PVC") or polyethylene terephthalate. As with
PDMS, polymer masks 40 may be selectively dipped in a solution
containing the photocatalyst, or the photocatalyst may, for
example, be painted, flame-coated or vapor deposited on the
surface. For polymers that are good transmitters, UV stabilizers
may be incorporated in/on the mask at all points outside of pattern
section 44, thereby forming mask section 42.
[0075] In a fourth embodiment, mask 40 comprises a glass. A
preferred technique for depositing photocatalyst layer 46 on the
glass is through chemical vapor deposition (CVD). As in FIG. 4B, an
additional shielding material may also be deposited. Additional,
alternative materials for forming mask 40, as well as additional
deposition techniques for forming mask section 42 and pattern
section 44, will be apparent to those of skill in the art.
[0076] When using a photocatalytic semiconductor layer 46, energy
source 50 preferably comprises a UV or x-ray lamp or laser. Energy
source 50 generates energy quanta above the band gap of
photocatalytic semiconductor layer 46. When using a photosensitizer
layer 46, energy source 50 preferably comprises a visible light
source, such as a light source with a wavelength between about
550-850 nm, for example, a visible laser light source, such as a
Helium Neon ("HeNe") laser. Energy source 50 is capable of exciting
photosensitizer layer 46. Other energy sources will be apparent to
those of skill in the art. Energy source 50 may be pulsed in order
to control an extent of radical generation and diffusion.
[0077] Medium M preferably comprises a medium capable of generating
radical species in the presence of electron hole pairs or excited
molecules, such as an oxygen- or nitrogen-containing medium. Medium
M may be either organic or inorganic and is preferably fluidic, for
example, a gaseous medium, a liquid medium, an aqueous medium, a
gel, water, or air. Furthermore, medium M preferably comprises an
oxidant, such as oxygen, nitrogen, oxidizing ions, Redox species,
Redox mediators, or electron transfer agents. The medium may also
or alternatively contain stabilizing agents, such as selenium,
zinc, lipoic acid, methionine, cysteine, or N,N Dimethyl glycine.
As yet another alternative, medium M may comprise more inert
conditions, such as vacuum or argon gas, in which case elements
capable of generating radical species are attached to substrate 30
or mask 40. Other mediums will be apparent to those of skill in the
art.
[0078] Referring to FIG. 5, in conjunction with FIGS. 2-4, a method
for using the apparatus of FIG. 4 is described. As seen in FIG. 5A,
mask 40 is brought into close proximity or contact with surface 34.
Energy source 50 is activated and irradiates mask 40 with incident
light 52. Mask section 42 of mask 40 inhibits incident light 52
from irradiating surface 34. However, where incident light 52
strikes pattern section 44 of mask 40, it generates radical
species. As discussed previously with respect to FIGS. 2A and 2B,
when photocatalyst layer 46 comprises a photocatalytic
semiconductor, electron hole pairs are generated within the
photocatalytic semiconductor because incident light 52 excites
layer 46 with energy above the band gap of the semiconductor. As
discussed previously with respect to FIG. 2C, when photocatalyst
layer 46 comprises a photosensitizer, incident light 52 excites the
photosensitizer in a manner capable of generating radical species
upon contact with appropriate molecules, for example, oxygen
molecules or thiohydroxamic esters.
[0079] The electron hole pairs or excited molecules generate
radical species R at the interface of medium M and layer 46.
Radical species typically are capable of traveling on the order of
100 nm. It is expected that radical species R will be transferred
from the interface of medium M and layer 46 to the interface of
medium M and oxidizable coating 36 of surface 34 along a
substantially shortest distance path. As seen in FIG. 5B, and
discussed previously with respect to FIG. 3, the radical species
locally oxidize coating 36 to form pattern 44' on surface 34 of
substrate 32. Pattern 44' replicates the shape of pattern section
44 of mask 40 on surface 34.
[0080] Such local patterning through chemical modification of
coating 36 is expected to alter the reactivity of the coating, and
may either stabilize or destabilize pattern 44'. Unaffected
adsorbed material optionally may be used for a second chemical
step, for example, a second masking step.
[0081] Coating 36 may, for example, be used in a manner similar to
the positive and negative resist coatings used in photolithography,
as discussed hereinabove with respect to FIG. 1. Thus, coating 36
may be removed at all points on surface 34 disposed within pattern
44', for example, via a secondary rinse. Alternatively, coating 36
may be removed at all points on surface 34 that are not disposed
within pattern 44'.
[0082] A significant advantage of the present invention, as
compared to prior art photolithography techniques, is that the
portion of pattern section 44 of mask 40 comprising photocatalyst
layer 46 is expected to enable patterning of features in coating 36
of surface 34 that are significantly smaller than the wavelength of
light generated by energy source 50. When using a photocatalytic
semiconductor, this is possible because the radical species
generated via photocatalytic semiconductor layer 46 have a
dimension on the order of sub-angstroms, as compared to incident
light 52, which has a dimension on the order of nanometers.
Likewise, when using photosensitizers, the radical species
generated with photosensitizer layer 46 are expected to be
significantly smaller than the wavelength of incident light. Thus,
pattern section 44 of mask 40 is preferably capable of patterning
surfaces with features having resolutions less than about 100 nm,
and even more preferably less than about 10 nm. Resolution of
pattern 44' may be controlled, for example, by controlling the size
of features within pattern section 44, and/or by controlling the
distance between mask 40 and surface 34.
[0083] Another significant advantage of the present invention is
that it is expected that the methods and apparatus described herein
may be used in conjunction with a variety of oxidizable surface
coatings 36. In a first embodiment, the surface coatings comprise
alkane thiols. Alkane thiols are described in greater detail in
U.S. Pat. Nos. 4,690,715 to Allara et al., 5,512,131 to Kumar et
al., 5,686,548 to Grainger et al., 6,020,047 to Everhart, 6,183,815
to Enick et al., and 6,048,623 to Everhart et al., all of which are
incorporated herein by reference. In a second embodiment, the
coatings comprise thioethers. Thioethers, including their oxidation
characteristics and their capabilities for selective modification,
are described in greater detail in U.S. patent application
Publication Ser. No. 2003/0,059,906 to Hubbell et al., as well as
pending U.S. patent application Ser. No. 10/246,362 to Hubbell et
al. (corresponding to PCT publication WO 03/024897), filed Sep. 18,
2002, and U.S. patent application Ser. No. 10/246,500 to Hubbell et
al. (corresponding to PCT publication WO 03/024186), filed Sep. 18,
2002, all of which are incorporated herein by reference. In a third
embodiment, the coatings comprise unsaturated materials, i.e.
materials comprising double or triple bonds. Coatings comprising
reactive saturated materials are also contemplated, for example,
materials comprising chlorine or bromine. In yet another
embodiment, the surface coatings comprise metallic oxides, or bare
metal substrates capable of oxidizing. Other coatings will be
apparent to those skilled in the art.
[0084] In an alternative embodiment of apparatus 30, mask section
42 of mask 40 is removed. As seen in FIG. 4C, photocatalyst layer
46 is deposited directly onto substrate 47 in a desired pattern,
thereby forming pattern section 44. Removal of mask section 42 is
significant in that many oxidizable surface coatings 36 would
spontaneously oxidize in the presence of incident light 52 of
adequate power. For this reason, mask section 42 provides shielding
in the embodiments of FIGS. 4A and 4B to ensure that energy of
incident light 52 only reaches surface 34 indirectly via radical
species generated in pattern section 44.
[0085] In this alternative embodiment, the energy and power of
incident light 52 generated by energy source 50 is specified such
that, when using a photocatalytic semiconductor, the excitation
energy delivered by incident light 52 is above the band gap of
photocatalytic semiconductor layer 46; alternatively, when using a
photosensitizer, energy delivered by incident light 52 is capable
of exciting photosensitizer layer 46 to a singlet state.
Furthermore, the excitation energy of incident light 52 preferably
is specified such that it is below the power typically required to
cause spontaneous oxidation of oxidizable surface coating 36. Thus,
incident light 52 that passes through mask section 42 of mask 40,
without contacting photocatalyst layer 46, irradiates coating 36
without causing oxidation. Oxidation still only occurs locally at
locations on surface 34 that are contacted by radical species
generated within pattern section 44, i.e. oxidation only occurs
within pattern 44' of surface 34.
[0086] When using a photocatalytic semiconductor layer 46, the band
gap energy of the photocatalytic semiconductor is dictated by:
E=hv (6)
[0087] where h is Plank's constant and equals
1.603.times.10.sup.-19, and E is the band gap energy of layer 46.
Since v is the frequency of incident light 52, and is related to
the wavelength .lambda. of the incident light by:
.nu.=C/.lambda. (7)
[0088] where C equals the speed of light, the excitation energy of
incident light 52 can be specified such that it is above the band
gap energy E of photocatalytic semiconductor layer 46 by choosing
an energy source 50 capable of generating incident light 52 of
appropriate wavelength. As an example, when layer 46 comprises
TiO.sub.2, the band gap energy is 3.2 eV, which may be generated by
the wavelength of light produced with either a UV or x-ray energy
source 50.
[0089] Next, it is believed that the power required for spontaneous
oxidation of coating 36 is dependent on Boltzmann's probabilistic
equation, which follows an exponential decay law such that, for the
purposes of the present invention, a probability of oxidation is
expected to decrease with decreasing power. By maintaining a power
level having a low probability of spontaneously oxidizing the
surface, it is expected that selective patterning may be achieved
with the alternative embodiment of mask 40 described hereinabove.
Reducing the amount of time which coating 36 is exposed to incident
light 52 may also reduce a probability of oxidation.
[0090] Although the equations above are believed to describe the
band gap energy of a photocatalytic semiconductor, and the
probability of a surface coating oxidizing in appropriate media
upon exposure to a given power level for a specified period of
time, the present invention is primarily concerned with the end
result, i.e. patterning. Thus, these equations are provided only
for the benefit of the reader and should in no way be construed as
limiting.
[0091] A significant advantage of the alternative embodiment of
mask 40 described with respect to FIG. 4C is that the criticality
of excluding incident light 52 from surface 34 is reduced. Thus,
increased flexibility is obtained in designing mask 40.
Furthermore, increased flexibility is obtained in specifying the
direction from which incident light 52 illuminates pattern section
44. This, in turn, increases flexibility in the positioning of
energy source 50. For example, in this alternative embodiment,
energy source 50 may illuminate pattern section 44 from the side,
from an angle, or from below mask 40, as compared to just from
above/through mask 40.
[0092] Referring now to FIG. 6, alternative embodiments of
apparatus in accordance with the present invention are described.
In FIG. 6A, as with apparatus 30, apparatus 100 comprises substrate
32 having surface 34 with oxidizable coating 36. Apparatus 100 also
comprises energy source 50 and medium M. Apparatus 100 still
further comprises stamp 110 having contact section 112 and pattern
section 114 with photocatalyst layer 116. As with apparatus 30,
when layer 116 comprises a photocatalytic semiconductor, energy
source 50 generates energy quanta above the band gap of the
photocatalytic semiconductor, and when layer 116 comprises a
photosensitizer, energy source 50 is capable of exciting the
photosensitizer. Oxidizable coating 36 and photocatalyst layer 116
are immersed in medium M. Stamp 110 is disposed between substrate
32 and energy source 50.
[0093] Contact section 112 is adapted to substantially contact
coated surface 34 at all points along the interface of stamp 110
with surface 34, except along pattern section 114. Contact section
112 preferably comprises a shielding material and/or stabilizing or
quenching agents on its underside at points that contact surface
34. However, contact section 112 may alternatively comprise a
material capable of transmitting incident light 52 generated by
energy source 50, or may comprise a partially transmitting
material.
[0094] When contact section 112 contacts coated surface 34, medium
M is preferably substantially excluded from the interface between
the contact section and the surface, thereby decreasing a
likelihood of spontaneous oxidation of coating 36 due to
irradiation with incident light 52. Pattern section 114 is
preferably slightly recessed with respect to contact section 112,
such that medium M remains in the interface between pattern section
114 and oxidizable coating 36 of surface 34, when contact section
112 contacts surface 34. The recession of pattern section 114 is
preferably less than about 100 nm, which is on the order of the
distance that radical species are able to travel.
[0095] FIG. 6B provides an alternative embodiment of apparatus 100
in which contact section 112 of stamp 110 is replaced with
transmission section 112', which is recessed with respect to
pattern section 114. Pattern section 114, meanwhile, substantially
contacts surface 34. In this embodiment, medium M remains in the
minute interface between surface 34 and pattern section 114, in
order to facilitate radical formation. It is expected that
oxidation efficiency may increase as a function of decreasing
distance between photocatalyst layer 116 and oxidizable coating 36.
Furthermore, if quenching species are disposed, for example, on the
underside of masking section 112', recession of section 112' may
decrease a likelihood of spontaneous oxidation of coating 36 via
transmission of incident light 52 through masking section 112'.
Alternatively, when transmission section 112' transmits incident
light 52, the light may be tuned such that it excites photocatalyst
layer 116, but does not induce spontaneous oxidation of coating 36
in the presence of medium M, as described hereinabove with respect
to FIG. 4C.
[0096] With reference now to FIG. 7, a method of using the
apparatus of FIG. 6A to selectively pattern surface 34 is
described. Although this method is described with respect to the
apparatus of FIG. 6A, it should be understood that a similar method
may be used with the apparatus of FIG. 6B, as will be apparent to
those of skill in the art. As seen in FIG. 7A, stamp 110 is brought
into contact with surface 34 such that contact section 112 of stamp
110 substantially excludes medium M from the interface between
contact section 112 and surface 34. Medium M remains in the
interface between pattern section 114 and oxidizable coating 36 of
surface 34. Energy source 50 is then activated and generates
incident light 52, which passes through stamp 110.
[0097] In pattern section 114, when photocatalyst layer 116
comprises a photocatalytic semiconductor, incident light 52
generates electron hole pairs within photocatalytic semiconductor
layer 116. When photocatalyst layer 116 comprises a
photosensitizer, incident light 52 excites the photosensitizer.
These electron hole pairs or excited photosensitizer molecules
generate radical species in the presence of medium M that are
transmitted to surface 34 and locally oxidize coating 36 to form
pattern 114' on surface 34. Pattern 114' replicates the geometry of
pattern section 114 of stamp 110 on surface 34, as seen in FIG.
7B.
[0098] In the preferred embodiment of contact section 112, the
contact section is shielded or quenched on its underside to prevent
incident light 52 from irradiating coating 36 at points where
contact section 112 contacts the coating. In an alternative
embodiment where contact section 112 is not, or is only partly,
shielded or quenched, incident light 52 passes through contact
section 112 and irradiates oxidizable coating 36 of surface 34.
Advantageously, even if the power of incident light 52 is
sufficient to spontaneously oxidize coating 36, since coating 36 is
substantially excluded from medium M at all locations along contact
section 112, the coating is unable to absorb the necessary
molecules required for oxidation, e.g. oxygen. Thus, coating 36
cannot oxidize at locations in contact with contact section 112
that are excluded from medium M, and it is expected that surface 34
may be selectively patterned regardless of whether contact section
112 transmits incident light 52.
[0099] As with apparatus 30, a significant advantage of apparatus
100 and all embodiments of the present invention, as compared to
prior art photolithography techniques, is that it is expected that
pattern 114' on surface 34 may contain features that are
significantly smaller than the wavelength of light generated by
energy source 50. This is possible because the radical species
generated via photocatalyst layer 46 have a dimension on the order
of sub-angstroms, as compared to incident light 52, which has a
dimension on the order of nanometers. Thus, pattern section 114 of
stamp 110 is preferably capable of patterning surfaces with
features having resolutions less than about 100 nm, and even more
preferably less than about 10 nm. Resolution of pattern 114' on
surface 34 may be controlled, for example, by controlling the size
of features within pattern section 114, and by controlling the
distance that pattern section 114 is recessed with respect to
contact section 112, thereby altering dispersion of radical
species.
[0100] Referring now to FIG. 8, yet another alternative embodiment
of apparatus in accordance with the present invention is described,
wherein the mask or stamp is replaced with a scanning probe. As
with apparatus 30 and 100, apparatus 150 comprises substrate 32
having surface 34 with oxidizable coating 36, as well as energy
source 50 and medium M. Apparatus 150 further comprises scanning
probe 160 having tip 162 with photocatalyst layer 164. Scanning
probe 160 is able to translate in directions 170, for example, the
X-, Y-, and/or Z-directions. Alternatively, directions 170 may
comprise the r-, .theta.-, and/or .PHI.-directions. Energy source
50 is coupled to tip 162 via coupling device 166, which may
comprise, for example, a fiber optic cable or a near-field optical
microscopy aperture. As previously, energy source 50 generates
energy quanta capable of exciting photocatalyst layer 164 of probe
160, and oxidizable coating 36 and photocatalyst layer 164 are
immersed in medium M.
[0101] With reference to FIG. 9, a method of using the apparatus of
FIG. 8 to selectively pattern a surface is provided. Scanning probe
160 is brought into close proximity or contact with surface 34, as
seen in FIG. 9A. Energy source 50 is activated, and incident light
52 travels through coupling device 166 to tip 162 of probe 160.
Incident light 52 excites photocatalyst 164 thereby forming radical
species in the presence of medium M, which are transmitted to
oxidizable coating 36 along a substantially shortest distance path.
Oxidizable coating 36 oxidizes locally at the point where these
radical species contact surface 34, i.e. at a point substantially
directly below tip 162 of probe 160, thereby forming selective
pattern 162' on surface 34, as seen in FIG. 9B. As discussed
previously, it is expected that the dimension of pattern 162'
advantageously may be significantly smaller than the wavelength of
incident light 52. Probe 160 may then be scanned or translated in
directions 170 while energy source 50 is activated to provide a
dynamic pattern 162', which may be specified by an operator in real
time, as seen in FIG. 9C.
[0102] The use of scanning probe 160 may be advantageous in some
applications because it provides highly localized oxidation of
surface 34. Additionally, the distance between probe tip 162 and
surface 34 may be finely adjusted to alter the resolution of
pattern 162', for example, by modulating dispersion of radical
species between tip 162 and surface 34. Furthermore, the resolution
of pattern 162' may be modulated by altering the cross-section of
layer 164 disposed on tip 162. Further still, by translating
scanning probe 160 in any plane, a vast variety of selective
patterns 162' may be provided on surface 34, i.e. a variety of
patterns may be oxidatively `carved` or `painted` into the surface.
An exemplary pattern 162' formed by translating scanning probe 160,
is provided in FIG. 9C. Probe 160 may be translated at any desired
rate, and/or with any desired power/energy parameters provided by
source 50. Additionally, energy source 50 may be intermittently
turned on and off, or pulsed, during translation of probe 160,
thereby providing a selective pattern 162' that is discontinuous
(see FIG. 10D). Moreover, an array of scanning probes may be
utilized, as is known in the lithographic arts.
[0103] Referring now to FIG. 10, a variety of exemplary selectively
patterned surfaces are provided. It is expected that these patterns
will be achievable using any or all of apparatus 30, 100, or 150
described previously, or with additional embodiments of the present
invention constructed in accordance with the present invention.
[0104] As discussed previously, local patterning of surface 34 of
substrate 32 via chemical modification of coating 36 is expected to
alter the reactivity of the coating, and may either stabilize or
destabilize the local pattern. Unaffected adsorbed material
optionally may be used for a second chemical step, for example, a
second masking step.
[0105] Furthermore, coating 36 may, for example, be used in a
manner similar to the positive and negative resist coatings used in
photolithography, as discussed hereinabove with respect to FIG. 1.
Thus, coating 36 may be removed at all points on surface 34
disposed within the local pattern, for example, via a secondary
wash, rinse, or etch. Alternatively, coating 36 may be removed at
all points on surface 34 that are not disposed within the local
pattern.
[0106] For the purposes of FIG. 10, patterns refer to portions of
coating 36 that have been removed from surface 34. In FIG. 10A,
surface 34 comprises local pattern 200 that was formed by a process
similar to a positive resist. In FIG. 10B, surface 34 comprises a
local pattern 202 that was formed by a process similar to a
negative resist. In FIG. 10C, which is shown in side-view, surface
34 comprises three-dimensional local pattern 204. Pattern 204 may
be formed, for example, by controlling an extent of oxidation of
coating 36 or by shaping surface 34 prior to patterning. In FIG.
10D, surface 34 comprises discontinuous local pattern 206. In FIG.
10E, surface 34 comprises two-step local pattern 208 having first
pattern 209 and second pattern 210. First and second patterns 209
and 210 may be formed, for example, with two separate masks or
stamps.
[0107] While preferred illustrative embodiments of the invention
are described hereinabove, it will be apparent to one skilled in
the art that various changes and modifications may be made therein
without departing from the invention. For example, the substrate or
surface on which the oxidizable coating is disposed may be provided
with a voltage bias, for example, an anodic bias, to facilitate
selective patterning of the surface. As another example, a mask or
stamp may be provided with two or more different photocatalyst
layers. When providing multiple photocatalytic semiconductor
layers, each may comprise a different band gap potential. When
providing multiple photosensitizer layers, each may comprise a
different excitation energy. A mixture of photocatalytic and
photosensitizer layers may also be provided. In such embodiments,
multiple energy sources may be provided, each capable of generating
energy at a different excitation level. Alternatively, a tune-able
energy source may be provided.
[0108] The mask or stamp may then be irradiated with incident light
of an energy capable of exciting the first photocatalyst layer, but
not the second, different layer. This creates a first pattern on a
target surface. A second pattern may then be provided by increasing
the excitation energy of the incident light generated by the energy
source to a level above the excitation energy of the second
photocatalyst layer, thereby creating a second pattern on the
target surface. Any number of patterns may be provided with this
technique using a single stamp or mask. Alternatively, multiple
masks or stamps may be used to generate multiple surface patterns
on a target surface. Further still, incident light may be exposed
to a photocatalyst layer in successive portions, thereby providing
multiple surface patterns from a single stamp or mask.
[0109] The appended claims are intended to cover all such changes
and modifications that fall within the true spirit and scope of the
invention. Additionally, it should be understood that, in order to
emphasize important aspects of the present invention, the FIGS. are
schematic and have not been drawn to scale.
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