U.S. patent application number 11/376908 was filed with the patent office on 2007-09-20 for pattern transfer by solid state electrochemical stamping.
Invention is credited to Nicholas X. Fang, Placid M. Ferreira, Keng Hao Hsu, Venkata K. Rapaka.
Application Number | 20070215480 11/376908 |
Document ID | / |
Family ID | 38510302 |
Filed Date | 2007-09-20 |
United States Patent
Application |
20070215480 |
Kind Code |
A1 |
Fang; Nicholas X. ; et
al. |
September 20, 2007 |
Pattern transfer by solid state electrochemical stamping
Abstract
The present invention provides an electrochemical fabrication
platform for making structures, arrays of structures and functional
devices having selected nanosized and/or microsized physical
dimensions, shapes and spatial orientations. Methods, systems and
system components of the present invention use an electrochemical
stamping tool for generating patterns of relief and/or recessed
features exhibiting excellent reproducibility, pattern fidelity and
resolution on surfaces of solid state ionic conductors and in
metal. Electrochemical stamping tools of the present invention are
capable high throughput patterning of large substrate areas and,
thus, enable a robust and commercially attractive manufacturing
pathway to a range of functional systems and devices including
nano- and micro-electromechanical systems, sensors, energy storage
devices and integrated electronic circuits.
Inventors: |
Fang; Nicholas X.;
(Champaign, IL) ; Ferreira; Placid M.; (Champaign,
IL) ; Hsu; Keng Hao; (Savoy, IL) ; Rapaka;
Venkata K.; (Champaign, IL) |
Correspondence
Address: |
GREENLEE WINNER AND SULLIVAN P C
4875 PEARL EAST CIRCLE
SUITE 200
BOULDER
CO
80301
US
|
Family ID: |
38510302 |
Appl. No.: |
11/376908 |
Filed: |
March 16, 2006 |
Current U.S.
Class: |
205/118 |
Current CPC
Class: |
B81C 2201/0154 20130101;
C25D 17/007 20130101; C25F 3/14 20130101; H01L 21/32134 20130101;
C25D 17/002 20130101; B23H 3/00 20130101; G01Q 80/00 20130101; B23H
3/04 20130101; C25D 5/02 20130101; B81C 1/0046 20130101; C25D
17/005 20130101; B82Y 30/00 20130101 |
Class at
Publication: |
205/118 |
International
Class: |
C25D 5/02 20060101
C25D005/02 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0001] This invention was made, at least in part, with United
States governmental support awarded by National Science Foundation
under contract number DMI-0328162. The United States government has
certain rights in this invention.
Claims
1. A method of making a structure comprising: a. providing a first
electrode in electrical contact with a solid state ionic conductor;
b. providing a second electrode in electrical contact with a metal;
c. establishing electrical contact between at least a portion of
said solid state ionic conductor and said metal; and d. generating
an electric field between said first and second electrodes, wherein
a portion of the metal is oxidized thereby generating metal ions
and free electrons, wherein said metal ions migrate through the
solid state ionic conductor to the first electrode where they are
reduced and wherein said free electrons migrate to said second
electrode, thereby making said structures; wherein said solid state
ionic conductor or said first electrode is a stamping tool that
generates a pattern of electrical contacts between said stamping
tool and said solid state ionic conductor or said metal.
2. The method of claim 1 wherein the metal comprises a metal layer,
a bulk metal, metal particles, metal cluster or a metal
substrate.
3. The method of claim 1 wherein the solid state ionic conductor
comprises metal atoms that are the same as metal atoms in said
metal.
4. The method of claim 1 wherein said electric field is generated
by applying a potential difference between said first and second
electrodes, wherein said second electrode has a higher electrical
potential than said first electrode.
5. The method of claim 4 wherein said potential difference between
first and second electrodes has a value selected from the range of
about 100 mV to about 2000 mV.
6. The method of claim 1 wherein said first electrode is a cathode
and wherein said second electrode is an anode.
7. The method of claim 1 wherein physical contact is established
between at least a portion of said stamping tool and said solid
state ionic conductor or said metal.
8. The method of claim 1 wherein said stamping tool comprises a
pattern of relief features, wherein physical or electrical contact
between at least a portion of said relief features and said solid
state ionic conductor or said metal generates said pattern of
electrical contacts.
9. The method of claim 8 wherein said pattern of said stamping tool
is at least partially transferred to said ionic conductor via
electrochemical etching or said metal via electrochemical
deposition.
10. The method of claim 1 further comprising the step of applying a
force to said stamping tool.
11. The method of claim 10 wherein said force is applied uniformly
to one or more surfaces of said stamping tool such that electrical
contact between at least a portion of said stamping tool and said
solid state ionic conductor or said metal is maintained during
processing.
12. The method of claim 1 wherein said ionic conductor is said
stamping tool, wherein said stamping tool has a selected pattern of
relief features, wherein physical contact between at least a
portion of said relief features and said metal generates said
pattern of electrical contacts.
13. The method of claim 12 wherein metal atoms are oxidized in
regions of said metal in physical contact with at least a portion
of said relief features of said stamping tool.
14. The method of claim 12 wherein localized electrochemical
etching of said metal occurs at regions of said metal in physical
contact with said relief features of said stamping tool.
15. The method of claim 12 wherein at least a portion of said
pattern of said stamping tool is transferred to said metal via
electrochemical etching.
16. The method of claim 12 wherein at least a portion of said
relief features of said stamping tool are nanosized relief
features, microsized relief features or both nanosized features and
microsized relief features.
17. The method of claim 1 wherein said first electrode is said
stamping tool, wherein said stamping tool has a shape selected such
that electrical contact between said stamping tool and said solid
state ionic conductor generates said pattern of electrical
contacts.
18. The method of claim 17 wherein said metal ions are reduced at
regions of said solid state ionic conductor in electrical contact
with said stamping tool, thereby generating one or more deposited
metal layers on a surface of said solid state ionic conductor in
electrical contact with said stamping tool.
19. The method of claim 17 wherein localized electrochemical
deposition of metal occurs at regions of said solid state ionic
conductor in electrical contact with said stamping tool.
20. The method of claim 17 wherein said stamping tool comprises a
plurality of features arranged in a selected pattern, and wherein
at least a portion of said pattern of said stamping tool is
transferred to a surface of said solid state ionic conductor via
localized electrochemical deposition.
21. The method of claim 20 wherein said features of said stamping
tool are nanosized features, microsized features or both.
22. The method of claim 20 wherein said features of said stamping
tool have substantially the same voltages.
23. The method of claim 20 wherein at least a portion of said
features of said stamping tool have substantially different
voltages.
24. The method of claim 20 wherein said stamping tool comprises an
array of individually addressable electrodes in electrical contact
with said solid state ionic conductor, wherein the voltage on each
electrode in the array is independently selectable.
25. The method of claim 24 wherein said stamping tool is capable of
transferring a pattern to a surface of said solid state ionic
conductor that is programmable, scalable or both programmable and
scalable.
26. An electrochemical patterning system for making one or more
structures, comprising: a first electrode in electrical contact
with a solid state ionic conductor; and a second electrode in
electrical contact with a metal, wherein at least a portion of said
solid state ionic conductor and said metal are in electrical
contact, wherein said solid state ionic conductor or said first
electrode is a stamping tool that generates a pattern of electrical
contacts between said stamping tool and said solid state ionic
conductor or said metal.
27. The system of claim 26 wherein said wherein solid state ionic
conductor and said metal are in electrical contact such that
generation of an electric field between said first and second
electrodes results in oxidation of a portion of said metal, thereby
generating metal ions and free electrons, wherein said metal ions
migrate through the solid state ionic conductor to the first
electrode where they are reduced and wherein said free electrons
migrate to said second electrode.
28. The system of claim 26 wherein at least a portion of said solid
state ionic conductor and said metal are in physical contact.
29. The system of claim 26 wherein said metal layer is said second
electrode.
30. The system of claim 26 further comprising an actuator
operationally connected to said stamping tool such that it is
capable of providing a force to said stamping tool that maintains
electrical contact between said stamping tool and said solid state
ionic conductor or metal.
31. The system of claim 26 wherein said stamping tool has a Young's
modulus selected from the range of about 20 GPa to about 200
GPa.
32. The system of claim 26 wherein said first electrode is a
cathode and said second electrode is an anode.
33. The system of claim 26 wherein said ionic conductor is said
stamping tool, wherein said stamping tool has a selected pattern of
relief features, wherein at least a portion said relief features of
said stamping tool are provided in physical contact with said
metal, thereby generating said pattern of electrical contacts.
34. The system of claim 33, wherein application of an electric
field between said first and second electrodes transfers at least a
portion of said pattern of said stamping tool to said metal layer
via electrochemical etching.
35. The system of claim 33 wherein at least a portion of said
relief features of said stamping tool are nanosized relief
features, microsized relief features or both nanosized features and
microsized relief features.
36. The system of claim 26 wherein said first electrode is said
stamping tool, wherein said stamping tool comprises a plurality of
features arranged in a selected pattern, wherein at least a portion
of said features are in electrical contact with said solid state
ionic conductor thereby generating said pattern of electrical
contacts.
37. The method of claim 36 wherein application of an electric field
between said first and second electrodes transfers at least a
portion of said pattern of said stamping tool to said solid state
ionic conductor via electrochemical deposition.
38. The system of claim 36 wherein said features of said stamping
tool have substantially the same voltages.
39. The system of claim 36 wherein at least a portion of said
features of said stamping tool have substantially different
voltages.
40. The system of claim 36 wherein said stamping tool comprise a
grid electrode.
41. The system of claim 36 wherein said stamping tool comprises an
array of individually addressable electrodes in electrical contact
with said solid state ionic conductor, wherein the voltage on each
electrode in the array is independently selectable.
42. The system of claim 36 wherein said stamping tool is
programmable, scalable or both programmable and scalable.
43. The system of claim 26 wherein said metal has a thickness
selected from the range of about a few nanometers to about 100 mm,
and wherein said solid state ion conductor has a thickness selected
from the range of about 50 nanometers to about 100 mm.
44. The system of 26 wherein said solid state ion conductor has an
ionic conductivity selected from the range of about 0.001 S/cm to
about 440 S/cm.
45. The system of 26 wherein said solid state ion conductor is
selected from the group consisting of Ag.sub.2S, Cu.sub.2S, AgI,
RbAg.sub.4l.sub.5, Ag.sub.3SI, AgCuS, AgCuSe,
Br.sub.4Cu.sub.16I.sub.7Cl.sub.13, and Cu.sub.2S.
46. The system of claim 26 wherein said solid state ion conductor
is an amorphous solid, a semicrystalline solid, a single
crystalline solid, or a composite material.
47. The system of claim 26 wherein said solid state ion conductor
is a superionic conductor.
48. The system of 26 wherein said metal is selected from the group
consisting of Ag, Cu, Au, Zn, and Pb.
49. An electrochemical stamping tool for etching structures into a
metal comprising: a first electrode having a first electric
potential; an ionic conductor having a selected pattern of relief
features, wherein said ionic conductor is in electrical contact
with said first electrode and wherein at least a portion of said
relief features are capable of establishing electrical contact with
said metal; and a second electrode in electrical contact with said
metal having a second electric potential that is higher than said
first electrode.
50. An electrochemical stamping tool for generating structures on a
solid state ionic conductor comprising: a first electrode
comprising a plurality of features arranged in a selected pattern,
wherein at least a portion of said features are capable of
establishing electrical contact with said solid state ionic
conductor; and a metal in electrical contact with said solid state
ionic conductor.
51. The electrochemical stamping tool of claim 50 wherein said
first electrode is an array of electrodes, wherein at least a
portion of the electrodes in the array are in electrical contact
with said solid state ionic conductor.
52. A method of making a structure comprising: a. providing a first
electrode in electrical contact with a metal and in electrical
contact with a solid state ionic conductor, wherein said metal
covers at least a portion of a surface of said solid state ionic
conductor; b. providing a second electrode in electrical contact
with said solid state ionic conductor; c. establishing electrical
contact between at least a portion of said solid state ionic
conductor and said metal; and d. generating an electric field
between said first and second electrodes, wherein metal atoms in
said metal are oxidized, thereby generating metal ions and free
electrons, wherein said metal ions migrate through said solid state
ionic conductor to said second electrode where they are reduced and
wherein said free electrons migrate to said first electrode,
thereby making said structures.
53. The method of claim 52 wherein said metal comprises a metal
surface, bulk metal, metal substrate, metal cluster or metal
particles.
54. The method of claim 52 wherein the electrical contact between
said first electrode and said metal is a single point contact.
55. The method of claim 52 wherein the electrical contact between
said first electrode and said metal is an electrical contact
pattern.
56. The method of claim 55 wherein said electrical contact pattern
is generated by a stamping tool.
57. The method of claim 55 wherein said electrical contact pattern
is generated by said first electrode having a plurality of features
arranged in a selected pattern, and wherein at least a portion of
said pattern is transferred to a surface of said metal via
localized electrochemical etching.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0002] Not Applicable.
BACKGROUND OF THE INVENTION
[0003] The use of solid state ionic conductors allows for
nano-scale patterning and stamping by highly localized
electrochemical etching and deposition. When an electric field is
applied by two electrodes in contact with a material that exhibits
ionic conduction, the metal ions near one of the electrodes migrate
through the bulk of the ionic conductor, and, upon receiving
electrons at the counter electrode, reduce back to metal atoms
precipitating at the interface. Alternatively, under a reverse
potential, a counter electrode of the metal is etched. By
nano-patterning the contact between the electrode and the ionic
conductor, one can deposit or etch metal patterns at a conductive
substrate.
[0004] Electrochemical micromachining, which works by local
dissolution of a conducting substrate (metals, semiconductors)
under an applied anodic bias in solution, shows promise in
fabricating 3D micro and nanoscale structures and devices, since it
requires relatively simple equipment and offers rapid etching
compared to other techniques such as ion beam milling and laser
abrasion. However, a liquid electrolyte, which is difficult to
handle, is required as a conducting medium between the two
electrodes. This challenge is overcome in the present invention by
utilizing solid state ionic conductors.
[0005] Terabe et al, demonstrate the use of mass transport in ionic
conductors to implement a quantized atomic conductance switch,
QCAS, where the concept of formation and dissolution of nanometer
silver cluster was used. In their QCAS, a silver wire with a thin
layer of silver sulfide cover was laid on a substrate, and a
platinum wire went across it with a gap of one nanometer [K.
Terabe, et al., Quantized conductance atomic switch, Nature, Vol
433, 6, Jan. 2005.]. By forming silver cluster from silver ions
drawn from underlying silver wire and hence bridging the gap in
between, the switch operated at room temperature at a frequency of
1 MHz.
[0006] Terabe et al, show formation and disappearance of nano scale
metal cluster on the apex of an Scanning Tunneling Microscopy (STM)
tip. Based on the concept of electrochemical reaction, they show
growth and shrinkage of a silver pillar of 70 nm in diameter and
200 nm in length on a silver sulfide coated silver STM probe [K.
Terabe, et al., Formation and disappearance of a nanoscale silver
cluster realized by solid electrochemical reaction, Journal of
applied physics, Vol 91, 12, Jun., 2002]. By controlling the
current going tunneling through the STM tip and their sample, the
growth rate of the silver cluster is regulated.
[0007] M. Lee et al. have used Atomic Force Microscopy (AFM), and a
super ionic conductor material, RbAg.sub.4I.sub.5, for
nanopatterning [M. Lee, et al., Electrochemical nanopatterning of
Ag on solid-state ionic conductor RbAg.sub.4I.sub.5 using atomic
force microscopy, Applied physics letters, Vol 85, 16, Oct. 2004].
With pulsed electric field input through a metal coated AFM probe
controlled to step across an RbAg.sub.4I.sub.5 sample, they were
able to place nanoscale silver cluster with each pulsed bias input,
and hence arrange the clusters in designed pattern.
[0008] The use of solid state ionic conduction for switches and for
single-point direct writing (with a modified stylus tip) has been
previously demonstrated.
[0009] None of these methods, however, are fully adaptable to
massive manufacturing due to the slow serial scanning process.
Accordingly, there is currently a need in the art for methods of
manufacturing structures, including nanostructures, that is capable
of high-throughput large area patterning. The invention disclosed
herein is a stamping process that can simultaneously produce a
number of spatial features and can scale-up to high production
rates for massive manufacturing over a large pattern area that
conventional approaches cannot match. An additional advantage of
the present methods and systems is the ionic stamp can be
programmed, scaled and reprogrammed with different metallic
nanopatterns for processes such as nano imprint lithography,
molding, transfer printing, etc. With appropriate solid
electrolytes, the processes disclosed herein can be used to
directly produce a structure or desired pattern of structures in
different metallic films, substrates, bulk materials or surfaces,
thereby saving steps compared to a conventional photolithography
patterning process.
SUMMARY OF THE INVENTION
[0010] The present invention provides an electrochemical
fabrication platform for making structures, arrays of structures
and functional devices having selected nanosized and/or microsized
physical dimensions, shapes and spatial orientations. Methods,
systems and system components of the present invention use an
electrochemical stamping tool for generating patterns of relief
and/or recessed features exhibiting excellent reproducibility,
pattern fidelity and resolution on surfaces of solid state ionic
conductors and in metal layers. Electrochemical stamping tools of
the present invention are capable high throughput patterning of
large substrate areas and, thus, enable a robust and commercially
attractive manufacturing pathway to a range of functional systems
and devices including nano- and micro-electromechanical systems,
sensors, energy storage devices and integrated electronic circuits.
Further, nanopatterning and micropatterning methods and systems of
the present invention are compatible with a wide range of
materials, including metals, metal alloys, ionic conductors and
superionic conductors, and processing conditions, including room
temperature (below about 30.degree. C.) processing.
[0011] In one embodiment, the present invention provides methods
for making structures, including nanostructures and
microstructures, using a stamping tool capable of pattern transfer
via electrochemical etching or electrochemical deposition. In a
method of the present invention, a first electrode is provided in
electrical contact with a solid state ionic conductor. A second
electrode is provided in electrical contact with a metal, such as a
metal film, substrate, surface, or bulk material, and optionally
the metal itself is the second electrode. Electrical contact and/or
physical contact is established between at least a portion of the
solid state ionic conductor and the metal, for example by a
configuration wherein the metal layer is separated from the first
electrode by the solid state ionic conductor. In this embodiment of
the present invention, the solid state ionic conductor or the first
electrode is a stamping tool that generates a pattern of electrical
contacts between the stamping tool and the solid state ionic
conductor or the metal. Optionally, this method of the present
invention may further comprise the step of applying a force to the
stamping tool, for example a force that is uniformly applied as a
function of a selected area of the stamping tool such that it
maintains electrical contact with at least a portion of the
stamping tool and the solid state ionic conductor or the metal
during processing.
[0012] To generate a structure or pattern of structures, an
electric field is established between the first and second
electrodes, for example by applying a selected potential difference
between first and second electrodes. Application of an electric
field results in oxidation of metal atoms in the metal and
subsequent migration of ions and electrons generated by the
oxidative process. In a useful embodiment wherein the second
electrode functions as an anode and the first electrode functions
as a cathode, oxidization generates free electrons that migrate
toward the electrode having a higher electric potential (i.e. the
anode) and mobile metal ions that migrate toward the counter
electrode (i.e. the cathode) having a lower electric potential. At
the counter electrode (i.e. the cathode) metal ions are reduced
back to metal atoms, for example by precipitation at the surface of
the counter electrode. The net effect of the oxidation-reduction
reactions and ion-electron transport processes is the formation of
structures by electrochemical etching of the metal or by
electrochemical deposition on a surface of the solid state ionic
conductor at the interface with the stamping tool. The present
invention, however, also includes patterning methods employing a
potential difference wherein the first electrode has a larger
electric potential than the second electrode. In this embodiment,
oxidation of metal deposits, particles or metals occurs at the
first electrode and reduction of metal ions occurs at the second
electrode. This aspect of the present invention may be used to
dissolve/reactively eliminate metals at the interface between the
solid state ionic conductor and the first electrode, in a manner
generating a structure or pattern of structures having selected
physical dimensions.
[0013] Transport of the metal ions through the solid state ionic
conductor is an integral process in the present invention and may
involve a transport mechanism involving conduction channels, grain
boundaries and/or the presence of bulk defects in the solid state
ionic conductor. In one embodiment, a potential difference between
first and second electrodes is established and maintained at a
value such that oxidation-reduction reactions occur at two
interfaces: (i) the interface between the solid state ionic
conductor and the metal layer and (ii) first electrode and the
solid state ionic conductor. Selection of the appropriate potential
difference in this aspect of the present invention depends on the
compositions, phases and oxidation-reduction chemistries of the
metal layer and solid state ionic conductor, and in some exemplary
embodiments range from about 100 mV to about 2000 mV.
[0014] In one embodiment of this aspect of the present invention, a
structure or pattern of structures are electrochemically etched
into the metal layer using a stamping tool that is the solid state
ionic conductor itself. In one embodiment, for example, an ionic
conductor-stamping tool is provided having a selected pattern of
relief features separated from each other by one or more recessed
regions. Patterns of relief features for ionic conductor-stamping
tools of the present invention may be generated by any means known
in the art including, but not limited to, optical lithograph,
electron beam writing, ion beam writing, soft lithograph, wet and
dry etching techniques and equivalents known in the art. Physical
contact between at least a portion of the relief features and the
metal generates the pattern of electrical contacts between the
stamping tool and the metal. In this embodiment, applying an
electric field results oxidization of metal in regions of the metal
in physical contact with at least a portion of the relief features
of the stamping tool. Metal ions generated via this oxidative
process migrate through the ionic conductor-stamping tool an
undergo reduction at the first cathode, thereby resulting in
localized electrochemical etching of the metal layer at regions of
the metal in physical contact with the relief features of the
stamping tool. This embodiment of the present invention provides a
means of at least partially transferring a pattern from the
stamping tool to the metal layer undergoing processing, for
example, by generating the negative relief pattern (i.e. the etch
pattern) of at least a portion of the pattern of relief features
into the metal layer.
[0015] In another embodiment of this aspect of the present
invention, a structure or pattern of structures are
electrochemically deposited onto a surface of the solid state ionic
conductor using a stamping tool that is the first electrode itself.
In one embodiment, for example, a first electrode-stamping tool is
provided that has a selected shape that generates a selected
pattern of electrical contacts between the first electrode-stamping
tool and a surface of the solid state ionic conductor undergoing
processing/patterning. Application of an electric field between a
first electrode provided at a lower electric potential and a second
electrode provided at a higher electrodic potential, results in
oxidation of metal atoms of the metal, thereby generating metal
ions that migrate to points of electrical contact in the pattern of
electrical contacts established between the first
electrode-stamping tool and the surface of the solid state ionic
conductor. In this method, reduction of metal ions at the interface
between the first electrode-stamping tool and the solid state ionic
conductor results in localized electrochemical deposition of metal
at regions of the solid state ionic conductor in electrical contact
with the stamping tool.
[0016] This embodiment of the present invention provides a means of
at least partially transferring a pattern from the first
electrode-stamping tool to the solid state ionic conductor
undergoing processing, for example, by reproducing the relief
pattern of at least a portion of the pattern of relief features
onto the surface of the solid state ionic conductor in electrical
contact with the stamping tool. Useful stamping tools of this
aspect of the invention include electrodes, shaped electrodes (e.g.
a grid electrode) and electrode arrays. In one embodiment, for
example the stamping tool comprises a shaped electrode having
plurality of features arranged in a selected pattern, such as a
grid electrode, wherein at least a portion of the pattern of the
shaped electrode is transferred to a surface of the solid state
ionic conductor via localized electrochemical deposition. In
another embodiment, the stamping tool comprises an array of
electrodes that may be held at substantially the same or,
alternatively, different electric potentials (i.e. voltages). In
another embodiment a programmable, scalable or reprogramable
electrochemical stamping tool is use comprising and array of
individually addressable electrodes in electrical contact with the
solid state ionic conductor, wherein the voltage on each electrode
in the array is independently selectable.
[0017] In methods of the present invention useful for certain
applications it is beneficial to use a combination of a metal and
solid state ionic conductor comprising metal atoms that having an
elemental composition that corresponds to that of the metal used
during processing. Use of a combination of elementally matched
metal and ionic conductor materials is useful because cations
generated from the metal generally will exhibit good transport
properties and conductance through the matched solid state ionic
conductor in the presence of an electric field, thereby allowing
for useful etch rates or deposition rates in the present methods.
The present invention includes methods, devices and systems using a
combination of a metal and solid state conductor that do not have
matched elemental composition with regard to the atomic composition
of the metal and the solid state ionic conductor. In these methods
and systems, therefore, the composition of the solid state ionic
conductor is selected such that it comprises an atom having an
elemental composition different from than that of the metal. In
these elementally unmatched metal and ionic conductor systems it is
useful to choose a metal that generates cations that are capable of
efficient transport through the solid state ionic conductor and
which exhibit appreciable solubility in the solid state ionic
conductor, such that appreciable etching rates and deposition rates
may be achieved.
[0018] The present methods are useful for patterning a wide range
of metal and solid state ionic materials. Metals and solid state
ionic conductors having planar surfaces, contoured (e.g. curved,
convex, concaved) surfaces, smooth surfaces, rough surfaces or any
combination of these may be patterned using the present methods,
devices and systems. The term "metal" is used expansively in the
present description and includes bulk metals, metal deposits, metal
films, metal substrates, metal particles, aggregates of metal
particles, metal clusters, and composite metal materials.
[0019] Another aspect the present invention provides patterning
systems using an electrochemical stamping tool capable of
electrochemical etching or electrochemical deposition for making a
structure or pattern of structures having selected physical
dimension, spatial orientation and positions. In one embodiment, a
system of the present invention comprises a first electrode in
electrical contact with a solid state ionic conductor; and a second
electrode in electrical contact with a metal. In this embodiment,
the solid state ionic conductor or the first electrode is a
stamping tool that generates a pattern of electrical contacts
between the stamping tool and the solid state ionic conductor or
the metal. Electrical contact and/or physical contact is
established between at least a portion of the solid state ionic
conductor and the metal, for example by a configuration wherein the
metal layer is separated from the first electrode by the solid
state ionic conductor. In a useful embodiment, for example, the
solid state ionic conductor and the metal are in electrical contact
such that generation of an electric field between the first and
second electrodes results in oxidation of metal atoms in the metal,
thereby generating metal ions and free electrons, wherein the metal
ions migrate through the solid state ionic conductor to the first
electrode where they are reduced and wherein the free electrons
migrate to the second electrode.
[0020] Useful stamping tools for certain embodiments of the present
invention have a Young's modulus selected from the range of about
20 GPa to about 200 GPa. A benefit of stamping tools of the present
invention having a Young's modulus in this range is that they are
less susceptible to pattern distortion than polymeric stamping
tools and stamps used in conventional soft lithography patterning
techniques, such as conventional nanoimprint lithography.
Accordingly, the methods, patterning systems and stamping tools of
the present invention are capable of providing good pattern
fidelity and high resolution patterning (e.g. resolution less than
about 100 nanometers, and more preferably for some applications
less than about 50 nanometers). An advantage provided by the
present methods, therefore, is the ability to use stamping tools
comprising solid state ionic conductor materials having a Young's
modulus selected over the range at about 20 GPa to about 200 GPa,
which are beneficial for minimizing or completely avoiding stamp
distortion during processing.
[0021] In an embodiment providing pattern transfer via
electrochemical etching, the ionic conductor is a stamping tool
having a selected pattern of relief features, wherein at least a
portion the relief features of the stamping tool are provided in
physical contact with the metal. This configuration provides a
pattern of electrical contacts that is useful for transferring at
least a portion of the pattern of the stamping tool (i.e. the
relief pattern) to the metal layer via electrochemical etching.
Useful stamping tools of this embodiment may have nanosized relief
features, microsized relief features or both, for example relief
features having nanosized lateral dimensions, nanosized vertical
dimensions or both. Use of nanosized and or microsized relief
features in this aspect of the present invention beneficial for
establishing electrical contact limited to selected nanosized
and/or microsized regions of the surface of the solid state ionic
conductor undergoing processing. This stamping tool configuration
is useful for generating nanosized and/or microsized structures and
patterns of nanosized and/or microsized structures
[0022] In an embodiment providing pattern transfer via
electrochemical deposition, the first electrode is a stamping tool
comprising a shaped electrode having a plurality of structural
features, such as a grid electrode, or an array of electrodes
provided in electrical contact with the solid state ionic
conductor. Electrode and electrode array geometries having
nanosized or microsized elements is beneficial for establishing
electrical contact limited to selected nanosized and/or microsized
regions of the surface of the solid state ionic conductor. This
stamping tool configuration is useful for generating nanosized
and/or microsized structures and patterns of nanosized and/or
microsized structures. Embodiments of this aspect of the present
invention also includes use of a scalable, programmable and/or
reprogrammable stamping tool comprising an array of individually
addressable electrodes, wherein the voltage on each electrode in
the array is independently selectable. Use of stamping tools
comprising individually addressable electrodes is useful for making
a wide range of structures, patterns and devices as the rate and
extent of electrochemical deposition on the solid state ionic
conductor surface can be individually and separately adjusted for
each electrode in the array, thereby providing a fabrication
pathway to structures and patterns of structures having a range of
physical dimensions.
[0023] An embodiment of the present invention is a method of
etching a metal layer. The method for making a structure comprises
providing a first electrode in electrical contact with a metal and
in electrical contact with a solid state ionic conductor, wherein
said metal surface covers at least a portion of a surface of said
solid state ionic conductor; providing a second electrode
electrically connected to a conductive material, including a metal,
metal surface, metal layer or bulk metal; establishing electrical
contact between at least a portion of said solid state ionic
conductor and said conductive material; and generating an electric
field between said first and second electrodes, wherein metal atoms
in said metal are oxidized, thereby generating metal ions and free
electrons, wherein said metal ions migrate through said solid state
ionic conductor to said second electrode where they are reduced and
wherein said free electrons migrate to said first electrode,
thereby making said structures. In an embodiment, the metal located
on a solid state ionic conductor is formed by one of the processed
disclosed herein.
[0024] The method can further comprise the first electrode that is
an anode and the second electrode that is a cathode.
[0025] In an embodiment, the electrical contact between said first
electrode and said metal is a single point contact. In an
embodiment, the electrical contact between said first electrode and
said metal is an electrical contact pattern. In an embodiment the
electrical contact pattern is generated by a stamping tool. In a
further embodiment, the electrical contact pattern is generated by
the first electrode having a plurality of features arranged in a
selected pattern, and wherein at least a portion of the pattern is
transferred to a surface of said metal via localized
electrochemical etching. In an embodiment, the metal surface is the
top surface of a metal layer having a depth or a thickness that
ranges between a few nanometers to bulk
[0026] The composition, physical state, and physical dimensions of
metal layers and/or solid ionic conductors of the present invention
are important parameters in patterning methods and systems of the
present invention. In a useful embodiment, the metal layer has a
thickness selected from the range of about a few nanometers to bulk
dimensions (e.g. greater than 1 micron), and the solid state ion
conductor has a thickness selected from the range of about 100
nanometers to about bulk dimensions (e.g. centimeters). Useful
solid state conductors have an ionic conductivity selected from the
range of about 0.001 S/cm to about 500 S/cm and include, but are
not limited to, Ag.sub.2S, Cu.sub.2S, AgI, RbAg.sub.4I.sub.5,
Ag.sub.3SI, AgCuS, AgCuSe, and Br.sub.4Cu.sub.16I.sub.7Cl.sub.13,
composite materials, materials that are amorphous solids,
semicrystalline solids or single crystalline solids. In some
embodiments of the present invention providing large etch rates or
deposition rates, a solid state ionic conductor is used having a
relatively large ionic conductivity, and in some embodiments of the
present invention providing small etch rates or deposition rates, a
solid state ionic conductor is used having a relatively small ionic
conductivity. The present methods and systems include use of solid
state ion conductors that are superionic conductors. Useful metals
for the methods and systems of the present invention include, but
are not limited to, Ag, Cu, Au, Pb, Zn, and other materials that
are conductive. In an embodiment, the metal composition matches the
metal composition of the solid state ionic conductor.
[0027] In another aspect, the present invention provides an
electrochemical stamping tools for etching structures into a metal
comprising: (i) a first electrode having a first electric
potential; (ii) an ionic conductor having a selected pattern of
relief features, wherein the ionic conductor is in electrical
contact with the first electrode and wherein at least a portion of
the relief features are capable of establishing electrical contact
with the metal; and (iii) a second electrode having a second
electric potential that is higher than the first electrode.
[0028] In another aspect, the present invention provides an
electrochemical stamping tool for generating structures on a solid
state ionic conductor comprising: (i) a first electrode having a
first electric potential; (ii) an ionic conductor having a selected
pattern of relief features, wherein the ionic conductor is in
electrical contact with the first electrode and wherein at least a
portion of the relief features are capable of establishing
electrical contact with a metal; and (iii) a second electrode
having a second electric potential that is higher than the first
electrode, wherein the second electrode is in electrical contact
with the metal or is the metal itself.
[0029] In another aspect, the present invention provides an
electrochemical stamping tool for generating structures on a solid
state ionic conductor comprising: (i) a first electrode comprising
a plurality of features arranged in a selected pattern, wherein at
least a portion of the features are capable of establishing
electrical contact with the solid state ionic conductor; and (ii) a
metal in electrical contact with solid state ionic conductor.
Optional, an electrochemical stamping tool of this aspect of the
present invention further comprises an electrode array, wherein
electrodes in the array are in electrical contact with the solid
state ionic conductor undergoing processing/patterning.
[0030] In another aspect, the present invention provides a method
of making a structure comprising the steps of: (i) providing a
first electrode in electrical contact with a solid state ionic
conductor; (ii) providing a second electrode in electrical contact
with a metal; (iii) establishing electrical contact between at
least a portion of the solid state ionic conductor and the metal;
and (iv) generating an electric field between the first and second
electrodes, wherein metal in the metal is oxidized thereby
generating metal ions and free electrons, wherein the metal ions
migrate through the solid state ionic conductor to the first
electrode where they are reduced and wherein the free electrons
migrate to the second electrode, thereby making the structures;
wherein the solid state ionic conductor or the first electrode is a
stamping tool that generates a pattern of electrical contacts
between the stamping tool and the solid state ionic conductor or
the metal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 provides a schematic diagram illustrating a general
system without electric potential (FIG. 1a), a deposition system
(FIGS. 1b-d), an etching system (FIG. 1e), and an electrochemical
patterning system (FIG. 1f) for generating nanostructures. In this
embodiment, the solid state ionic conductor is Ag.sub.2S, the metal
is silver, and the anode and cathode are connected to a power
supply with reversible polarity so that the location of the anode
and cathode change from the bottom and top, respectively (FIGS.
1b-d) to the top and bottom (FIG. 1e). FIGS. 1b-d illustrate
building of nanostructures by deposition. FIG. 1e illustrates
reversing the electric potential builds nanostructures by etching
metal overlaying the solid state ionic conductor. The direction of
the current is indicated by the arrows showing the flow of
electrons ("e.sup.-").
[0032] FIG. 2 provides atomic force microscopy (AFM) micrographs
illustrating the writing and dissolution of silver structures using
the scheme depicted in FIGS. 1b-e using an AFM electrode. FIG. 2
comprises five frames: frame 1 is an AFM photograph the surface of
the Ag.sub.2S before the process; frames 2-5 are AFM images of the
surface after each line in the asterisk is drawn.
[0033] FIG. 3 provides atomic force microscopy (AFM) photographs
(FIGS. 3A & C) and corresponding height measurements (FIGS. 3B
& D) illustrating the dissolution and growth of silver
structures. The lines in A and C track the positions at which
heights are measured in B and D. The silver clusters are written to
a height of about 250 nm (see B and D) and dissolved to a height of
about 150 nm (see A and C).
[0034] FIG. 4 shows AFM images of electrochemical stamping of
silver structures on Ag.sub.2S with a stamping tip using the scheme
shown in FIG. 1f. The top panel (A) shows the Ag.sub.2S surface
prior to stamping, the middle panel (B) shows one stamped
nanostructure (see structure within the highlighted circle), and
the bottom panel (C) three replicated nanostructures.
[0035] FIG. 5 shows optical images of large-area electrochemical
stamping with micrometer resolution. The top panel (A) shows the
silver sulfide stamping tool and the bottom panel (B) shows the
etched silver film. The bar is 200 .mu.m.
[0036] FIG. 6 is a photograph of a system for electrochemical
stamping. Positioning stages are labeled (a) and (b). Electrodes
are labeled (c) and (e). Optical microscopy for process monitoring
is labeled (d).
[0037] FIG. 7 is a time-lapsed sequenced of optical microscope
image of solid-state electrochemical stamping as produced by the
system of FIG. 6 using a silver sulfide stamping tool on a Ag
surface on chrome on glass. A is prior to stamping; B is an
intermediate stage; and C is when stamping is substantially
complete.
[0038] FIG. 8 is a cyclic voltammetry characterization of a silver
sulfide stamping tool to determine typical redox potential of
Ag/Ag.sup.+. The black lines are ramping up and the red lines are
ramping down, as indicated in the legend.
[0039] FIG. 9 provides a series of current as a function of time
for metal etching with a silver sulfide stamping tool for four
different voltages as indicated.
[0040] FIG. 10A provides an AFM image of three Ag clusters Ag
clusters (identified by number 1, 2 and 3 in the bottom panel)
drawn to the surface of a Ag.sub.2S film by means of a charged AFM
tip. The three clusters have similar topography. The top panel is
the surface of the Ag.sub.2S film before and the bottom panel after
application of an electric potential. The bar is 1 .mu.m. FIG. 10B
provides a schematic illustration of a transfer stamping process,
where a programmable Ag.sub.2S stamping tool (panel a) is brought
into contact with Ag film surface and a potential difference
between the cathode and anode applied (panel b) to selectively etch
Ag substrate or Ag film to provide a three-dimensional profile on
the surface (panel c).
[0041] FIG. 11 provides experimental characterization of superionic
conduction at nanoscale. FIG. 11A provides a graphical
representation of cyclic voltammetry to monitor the stamp etching
process conditions. FIG. 11B is an SEM image of a cross-section of
the Ag.sub.2S stamping tool, revealing directional formation of
silver nanowires at a scale <100 nm.
[0042] FIG. 12 provides an overview of a stamping process. FIG. 12A
is a schematic showing a cathode-anode pair, with an Ag.sub.2S
shaped stamping tool in electrical contact with, and positioned
between, the cathode and Ag film. FIG. 12B is an SEM image of an
Ag.sub.2S stamping tool prepared by Focused Ion Beam (FIB) milling
(scale bar 5 um). FIG. 12C is an SEM image of a sub-micron line
etched out of an Ag metal (scale bar 5 um).
[0043] FIG. 13 provides a schematic for a self-sustaining
nano-ionic system that use patterns made by the methods disclosed
herein.
[0044] FIG. 14 provides a schematic of an electronically
reconfigurable plasmonic switch using growth and dissolution of Ag
nanodots. When the polarity of the switch reversed (compare FIGS.
14A and 14B), the optical lightwaves are guided to the
corresponding branches, as the overgrowth of nanodots experience a
red-shift in wavelength and reject the light signal. The top panel
of each of A and B are side views, and the bottom panels are top
views.
[0045] FIG. 15A is an SEM image of a Ag.sub.2S stamping tool of the
present invention. FIG. 15B is an SEM image of the corresponding Ag
film electrochemically stamped by the stamping tool of FIG. 15B.
The scale bar is 1 um.
[0046] FIG. 16 provides a schematic illustration of etching a metal
with an electrochemical stamp or stamping tool. FIG. 16A shows a
metal (Ag) and a solid state ionic conductor (Ag.sub.2S) having a
three-dimensional surface (e.g. the stamp) that are not in
electrical or physical contact. FIG. 16B shows the stamping
process, wherein only a portion of the stamp surface and metal are
in electrical contact. There is a detailed view of the boxed region
showing that where the Ag.sub.2S and Ag are in contact, oxidation
of metal on the metal surface occurs, but substantially no
oxidation occurs on the metal surface that is not in contact with
the Ag.sub.2S conductor. After the stamping process is complete,
the stamp is removed from the metal leaving a three-dimensional
pattern in the surface of the metal, as shown in FIG. 16C.
[0047] FIG. 17 provides cyclic voltametry plots of the silver
sulfide stamp measured between two silver electrodes.
[0048] FIG. 18 provides plots of current as a function of time for
various driving potentials during an etching process.
[0049] FIG. 19 is a pair of SEM images showing a silver sulfide
stamp (A top view; B perspective view) and the corresponding
produced silver feature (C) etched from a silver film.
[0050] FIG. 20 is a plot of lateral width reduction as a function
of line width for four driving potentials indicating that for the
smallest line width stamp (110 nm), a driving potential of 0.6V
provides the lowest lateral width reduction so that the feature is
reduced by 13% (e.g. 95 nm width).
[0051] FIG. 21 is a plot of etch rate (nm/s) and surface roughness
(nm) as a function of the driving potential (volts).
[0052] FIG. 22 is an SEM photograph of an etched silver metal made
with an electrochemical Ag.sub.2S stamping tool patterned with a
5.times.5 plasmonic array.
[0053] FIG. 23 provides SEM images showing the resolution of
patterns etched into silver metal using an Ag.sub.2S
electrochemical stamping tool of the present invention. Two regions
of the etched surface are expanded to show the invention provides
for pattern creation with line spacing of 50 nm and lower and
lateral resolution of 60 nm and better.
DETAILED DESCRIPTION OF THE INVENTION
[0054] The invention may be further understood by the following
non-limiting examples. All references cited herein are hereby
incorporated by reference to the extent not inconsistent with the
disclosure herewith. Although the description herein contains many
specificities, these should not be construed as limiting the scope
of the invention but as merely providing illustrations of some of
the presently preferred embodiments of the invention. For example,
thus the scope of the invention should be determined by the
appended claims and their equivalents, rather than by the examples
given.
[0055] As used herein, "structure" is used broadly to refer to
formation of patterns, including recessed, relief, or a combination
of recessed and relief patterns. A recessed pattern refers to a
pattern that is formed by etching a surface, such that channels
and/or depressions are formed on the surface. This is also commonly
known as "top-down" manufacture. A relief pattern is one that is
formed by deposition of material onto a surface to form a pattern.
This is also commonly known as "bottom-up" manufacture. The
structure can be a three-dimensional pattern, having a pattern on a
surface with a depth and/or height to the pattern. Accordingly, the
term structure encompasses geometrical features including, but not
limited to, any two-dimensional pattern or shape (circle, triangle,
rectangle, square), three-dimensional volume (any two-dimensional
pattern or shape having a height/depth), as well as systems of
interconnected etched "channels" or deposited "walls." In an
embodiment, the structures formed are "nanostructures." As used
herein, "nanostructures" refer to structures having at least one
dimension that is on the order of nanometers to microns. In an
embodiment the nanostructure has at least one feature that is on
the order of tens of nm. For example, the width of the line can be
on the order of 10's to 100's of nm and the length can be on the
order of microns to 1000's of microns. In an embodiment the
nanostructure has one or more features that range from an order of
tens of nm to hundreds of nm.
[0056] A "pattern of structures" refers to a plurality of
structures that are deposited and/or etched on a surface by a stamp
or stamping tool. Accordingly, the term encompasses a plurality of
geometrical features etched onto a surface, as well as a plurality
of geometrical features deposited onto a surface. The present
methods and system are capable of generating patterns of structures
having well defined and selected physical dimensions, spatial
orientations and positions.
[0057] A "stamp" or a "stamping tool" refers to a material having a
surface that is shaped for etching and/or depositing a pattern of
structures. Accordingly, the stamping tool can have one or more
recessed features and/or one or more relief features that define
the stamp's "shaped surface." The stamping tool facilitates pattern
transfer from the stamp surface. The stamp's "shaped surface" is a
three-dimensional shape on the surface that makes electrical
contact with a metal surface and, in particular, an electrical
contact that is a "pattern of electrical contacts." In an
embodiment, the composition of the stamping tool comprises a
solid-state ionic conductor. In an embodiment, the stamping tool
comprises one or more features on an electrode. A feature on an
electrode is a shape that provides an electrical contact pattern.
Depending on the process, and in particular the direction of the
electric field (e.g. relative electric potentials of the
electrodes), the stamping tool can deposit metal structures on a
substrate surface to make a relief pattern of structures, or the
stamping tool can etch a metal surface to make a recess pattern of
structures that correspond to the stamp relief features. In an
embodiment, the generated structure comprises both a relief
structure and a recess structure. The stamping tool relief features
can be constructed by methods known in the art, including by
focused ion beam milling. The surface of the stamp that makes
electrical contact with a conducting surface can have any shape,
including substantially planar, curved, or a combination of planar
and curved,
[0058] The dimensions of the relief feature can be microsized,
nanosized, or both microsized and nanosized. A feature is
microsized if it has dimensions on the order of greater than
microns. A feature is nanosized if it has any one or more
dimensions on the order of less than about one micron. In an
embodiment, a nanosized feature is less than about 100 nm. A
"lateral dimension" refers to a distance that is parallel to the
interaction surface of the stamping tool and solid ionic conductor
or the stamping tool and the metal. A "vertical dimension" refers
to the height of the relief feature.
[0059] "Electrical contact" refers to the configuration of two or
more elements such that a charged element is capable of migrating
from one element to another. For example, a cathode in electrical
contact with a solid state ionic conductor permits a metal ion to
migrate from the interior of the solid state ionic conductor to the
region between the surface of the cathode and the surface of the
conductor, where the metal ion is reduced. Similarly, an anode in
electrical contact with a metal permits free electrons released due
to metal atom oxidization to flow from the metal to the anode.
Accordingly, electrical contact encompasses elements that are in
"physical contact." Elements are in physical contact when they are
observable as touching. Electrical contact also includes elements
that may not be in direct physical contact, but instead may instead
have an intervening element, such as an electrolyte or a conductive
material, located between the two or more elements. Accordingly,
electrical contact encompasses an electrode and a solid state ionic
conductor, wherein metal is deposited and reduced between the
surface of the electrode and the solid ionic conductor.
[0060] "Pattern of electrical contacts" refers to a pair of
surfaces that have regions of electrical contact and regions of no
electrical contact. For example, in the processes disclosed herein,
a stamping tool of the present invention is said to have a "pattern
of electrical contacts" with a metal so as to generate an etched
structure. In an embodiment, the pattern of electrical contacts
corresponds to a pattern of physical contact between the stamping
tool and the surface to be etched. In an embodiment, the pattern of
electrical contacts corresponds to a pattern of physical contact
between the stamping tool and the substrate surface on which the
deposited metal rests. The process of reducing ionized metal atoms
at the interface between the stamp and solid state ionic conductor
is referred to as "electrochemical deposition." The process of
oxidizing metal at the physical contact pattern between the
stamping tool and the metal surface is referred herein as
"electrochemical etching." Accordingly, the stamp or stamping tool
is also referred herein as an "electrochemical stamp," wherein the
stamping tool can be used to etch or deposit metal.
[0061] "Localized electrical deposition" refers to deposition that
is substantially restricted to an area defined by a region between
the stamping tool and the solid state ionic conductor. Outside this
region, substantially no reduction of ions, and corresponding
deposition, occur. In an embodiment, substantially no reduction
refers to a region outside the physical contact area between the
stamping tool and metal or stamping tool and solid state ionic
conductor.
[0062] The stamp and/or the stamping tool has mechanical attributes
and characteristics, including Young's modulus, compressibility
modulus, conductivity, flexural rigidity, that are optimized as
known in the art to ensure suitable structures are obtained from
any of the processes disclosed herein. In an embodiment, a separate
element such as a rubber or other elastomeric material, is
incorporated into a stamping tool to ensure that as the deposition
and/or etching process proceeds, physical contact is maintained
between the stamp and surface during etching and/or deposition. In
an embodiment, a force actuator is connected to the stamping tool
for applying a constant and uniform force, and corresponding
pressure, between the stamping tool and solid state ionic conductor
or metal throughout processing. A force is said to be uniformly
applied to a surface such that the pressure distribution between
the stamping surface and metal is substantially uniform, thereby
ensuring the stamping tool remains level relative to the metal. In
other words, the etch rate is uniform over the metal surface, and
is independent of location on the metal surface. In addition, a
uniform force ensures continued physical contact between the
stamping tool and the etched metal throughout the etching
process.
[0063] "Cathode" and "anode" have their art-recognized meanings. An
anode is an electrode where oxidation occurs and a cathode is where
reduction occurs. An anode and cathode form an electrode pair
where, when each are charged to different electric potentials and
used in a process disclosed herein, redox reactions occur. The
cathode and anode are made from materials known in the art. In an
embodiment the cathode and anode are platinum. The electrodes are
each electrically connected to a power supply, so that electrons
generated at the anode travel to the cathode.
[0064] A "solid state ionic conductor" refers to a material that is
in a solid-state and can conduct ions. The solid state ionic
conductor functions as a membrane that separates the anode from the
cathode, such that at least a portion of the oxidized metal travels
from the anode, through the solid ionic conductor, to the cathode
surface. Preferred solid state ionic conductors have the property
of being fast and selective conductors of a metal ion. The solid
state ionic conductor has an ionic conductivity so that patterned
structures are obtained. For example, the ionic conductivity can be
between about 0.001 to 500 S/cm.sup.2, wherein the ionic
conductivity is selected so as to obtain a desired etch rate. The
solid state ionic conductor includes any materials that are
solid-state and selectively conduct metal ions. For example, the
solid state ionic conductor encompasses materials that are
amorphous solids, have grain boundaries, electroactive polymers,
composites and/or comprise single crystalline materials. Polymers
and glasses can also comprise solid state ionic conductor. The
solid state ionic conductor can comprise a composite material
having a mobile ionic conductive phase embedded in a host matrix.
Useful solid state ionic conductors of the present invention
include a mobile ionic conductive phase in a polymer or glass host
matrix and include nano particle composite materials. The solid
electrolyte can comprise those disclosed in U.S. Pat. Pub. No.
2003/0044687 (a first binding polymer and a second polymer composed
of alkali metal ion conducting polymers), U.S. Pat. No. 6,165,705
(MAg.sub.4I.sub.5, where M is a monovalent cation), and others
known in the art, including but not limited to, Ag.sub.2S, AgI,
RbAg.sub.4I.sub.5, Ag.sub.3SI, AgCuS, AgCuSe,
Br.sub.4Cu.sub.16I.sub.7Cl.sub.3, and Cu.sub.2S.
[0065] "Potential difference" refers to a cathode and anode having
different electric potentials to generate an electric field, such
that electrons migrate to the anode, and ions selectively migrate
from the anode to the cathode via a path through the solid state
ionic conductor positioned between the anode and cathode.
[0066] "Metal," "Metal film" or "metal layer" refers to a metal
material having a surface where oxidation and/or reduction may
occur. In an embodiment, the metal is an integral part of the
electrode such that the metal is at least a portion of the
electrode. In an embodiment, the metal is a metal surface of a
metal film, bulk metal, metal substrate, metal particle, metal
cluster, metal composite or metal layer that is electrically
connected to the electrode. In an embodiment, the metal is a bulk
metal. "Bulk metal" refers to a metal that is shaped so that it has
dimensions on the order of microns and higher. A dimension referred
to as "bulk" has a length on the order of microns and higher. In an
embodiment the metal is adjacent and covers at least a portion of a
substrate. In an embodiment, the substrate provides structural
support to a metal and assists in positioning the metal relative to
the counter electrode or the stamping tool. In an embodiment, the
substrate comprises chrome or glass. In an embodiment, the
substrate comprises a translucent material, or a window, to assist
in optical visualization of the process. In an embodiment, the
substrate is a solid state ionic conductor. In an embodiment, the
thickness of the metal layer is between about 10 nm and 5 mm. In an
embodiment, the thickness of the metal layer is between 10 nm and 1
.mu.m. In an embodiment, the thickness of the metal layer is
between 10 nm and 500 nm. In an embodiment, the thickness of the
metal layer is about 200 nm. In an embodiment the metal layer
comprises Ag, Au, or Cu. In an embodiment, the metal layer is
Ag.
[0067] A metal ion is said to "migrate through" the solid ionic
conductor under an electric potential when the metal ion travels
from the surface of the metal in electrical contact with the anode
to the surface of the cathode by a path within the solid ionic
conductor.
[0068] An "individually addressable electrode" refers to an
electrode that comprises an array of electrodes, wherein each
member of the array is independently controllable. Independently
controllable refers to an electrode having a potential that can be
varied independently of the potential of other electrode array
members. An individually addressable electrode is accordingly
reprogrammable and reconfigurable, such that a single stamp or
stamping tool can be used to generate different structures, and
provide a user more control over generated structures. Individually
addressable electrodes permit pattern transfer that is programmable
and/or scalable. A programmable, reprogrammable and/or scalable
electrode array permits a single stamping tool to be variable
configurable such that a single stamp can create any number of
patterns by electronically controlling the electric potential
distribution across the surface of the stamp. A programmable,
reprogrammable and/or scalable electrode array is capable of
generating different, independently selectable patterns on surface
or in materials using the same stamping tool.
[0069] A stamping tool is said to have features of "substantially
the same voltages" when there is less than about 5% voltage
variation between features, including less than about 1% voltage
variation between features. A stamp having features of
"substantially different voltages" refers to a voltage variation of
any one or more feature being greater than 1%, including greater
than 5%.
[0070] In the following description, numerous specific details of
the devices, device components and methods of the present invention
are set forth in order to provide a thorough explanation of the
precise nature of the invention. It will be apparent, however, to
those of skill in the art that the invention can be practiced
without these specific details.
[0071] This invention provides methods for making patterns,
including micropatterns, nanopatterns and a combination of micro
and nanopatterns. The present invention provides methods of
patterning by electrochemical stamping, to provide relief and/or
recess features directly to a metal surface or metal overlaying a
substrate surface, wherein the substrate surface is a solid state
ionic conductor.
[0072] FIG. 1 provides a schematic diagram illustrating a side view
of a pair of electrodes, between which lie a solid state ionic
conductor (e.g. Ag.sub.2S) and a metal (e.g. Ag). A distinct
property of solid-state ionic conductors is that an electric field
can induce ion migration resulting in mass transport, providing the
mechanism of electrochemical deposition and etching, including
structures having nanoscale to microscale dimensions. Some examples
of solid-state ionic conductors are copper sulfide (Cu.sub.2S),
silver iodide (AgI), silver sulfide (Ag.sub.2S), etc. FIG. 1 shows
the basic schemes for electrochemical patterning. A silver film 20
(connected to one electrode 30) is separated from the counter
electrode 30 by a solid electrolyte 10, silver sulfide for example.
Under an electric field generated by a power supply 35 (see FIG.
1(b)), silver atoms at the silver substrate 20 are oxidized into
silver ions 42 and electrons 44. While electrons move to the anode
electrode 40 and then the cathode electrode having higher potential
50, mobile silver ions 42 migrate through the conduction channels
formed by the accumulation of defects in the ionic conductor bulk
10 to the counter electrode 50 where, with the available electrons,
they are reduced back to silver atoms 60 (see FIG. 1(c)) to form a
metallic nanostructure. This is depicted in FIGS. 1 (b)-(d). The
bias between sample 20 and electrode 50 is such that the redox
reaction takes place at two interfaces, one between silver sulfide
10 and the underlying silver film 20, the other between electrode
50 and silver sulfide substrate 10.
[0073] FIGS. 1 (d)-(f) depict producing metallic nanostructures by
a writing tool 68. FIG. 1(d) shows the writing feature of a sharp
tip electrode 68 that deposits Ag 60 on an Ag.sub.2S substrate
surface 10. FIG. 1(e) shows the erasing feature of a sharp tip
electrode by moving the sharp tip electrode under a reverse
electric field such that the cathode 50 and anode 40 reverse
relative to writing scheme depicted in FIGS. 1(b)-(d), such that a
portion of the metal layer 60 is erased. FIG. 1(f) illustrates the
use of a stamp 70 connected to a cathode 50 for single-step
production of a patterned deposited structure 60 on the surface of
the solid state ionic conductor 10.
[0074] FIGS. 2 and 3 show AFM images of the results obtained using
the process summarized in FIG. 1. FIG. 2 demonstrates use of the
process to write a silver asterisk by a sequence of lines as shown.
Panel 2 contains a single silver line, panel 3 contains two lines
forming an X shape, Panels 4 and 5 show the three lines forming an
asterisk. FIG. 3 shows the growth and dissolution of a silver
pillar by reversing the electric field. The silver structure was
grown to 250 nm (FIGS. 3C and 3D) and then dissolved to 150 nm
(FIGS. 3A and 3B).
[0075] FIG. 4 shows the results for a process using silver metal
and a silver sulfide solid state ionic conductor. A silver
microstructure is circled in FIG. 4B. Additional silver
microstructures are drawn out of the silver sulfide sample by a
charged AFM tip, with three such structures shown in FIG. 4C.
[0076] Stamping experiments implement the concept depicted in FIG.
1(f), wherein an electrode with a desired pattern is brought into
contact with a metal surface. With an electric field generated
between the anode and the cathode and across the solid ionic
conductor, repeated metal structures are drawn out of the ionic
conductor and deposited on its surface. In an alternate embodiment,
a pre-patterned solid state ionic conductor stamp is placed in
physical contact with a metal, and electrical potential with the
correct polarity applied (e.g. silver having a higher potential
than the anode that is connected to the stamp). Under this
electrical potential, the metal atoms on the substrate in immediate
contact with the stamp are ionized into mobile metal ions that
migrate into the stamp and free electrons that move through the
remaining metal to the anode. Accordingly, only the portions of the
metal in contact with the stamp are etched. The etching process
proceeds until substantially all the metal atoms making up the film
are oxidized and absorbed into the stamp, revealing an optional
underlying chrome film or the process is ended by terminating the
applied electric field. Any connection, so long as continued
conductivity is maintained during the process, is encompassed by
the present invention.
[0077] FIG. 5 is an optical microscopy image of one such patterning
process. FIG. 5A shows the pattern of the silver sulfide stamp.
FIG. 5B is the corresponding pattern etched into the surface of a
silver film. The stamp is cut out of a silver sulfide crystal and
pressed between two flat parallel surfaces to maximize the contact
between the silver film and the stamp. The stamp is pre-patterned
with a grating pattern on one side, and secured to a platinum
electrode on the other, and mounted on a column. The patterned face
of the stamp is brought in contact with a glass substrate having a
100 nm layer of silver deposited over a layer of chrome using a
mild pressure and corresponding force. An elastomer material (see
element 75 in FIG. 12A) is optionally inserted between the column
and the electrode and initially compressed by the pressure between
the stamp and the contact. This elastomeric material serves as a
compliant mechanism that gradually relaxes to maintain good contact
between the stamp and the substrate as the silver layer is etched
during the electrochemical process. Application of a constant force
or pressure ensures steady contact is maintained between the stamp
and the metal thereby maximizing the resolution and repeatability
of generated structures. Suitable elastomeric materials including,
for example, rubber, are commercially available. Commercially
available polymer-based visco-elastic material include, for
example, Sorbothane.RTM., and PDMS or other silicone rubbers. In an
embodiment, the force actuator comprises an compressible
elastomeric material.
[0078] FIG. 6 shows a system used for an etching procedure. An
optical microscope is placed underneath the glass sample stage to
image the bottom surface of the glass sample. The entire etching
process is monitored by observing the color change of the contact
area between the film and the stamp due to the change of the
thickness of the silver film. Electric current and time are also
monitored and synchronized with the digital video captured through
the microscope. This combination of quantitative (current and time)
and qualitative data acquisition (video image, color monitoring)
allows visual events to be matched with current changes, thereby
permitting a more in-depth analysis of the electrochemical process.
The color of the area against which the stamp is pressed changes
gradually, as observed by the optical microscope, indicating the
change of the thickness of the silver film as etching proceeds. The
final color when etching is complete is blackish, as shown in FIG.
7.
[0079] FIG. 7 is a series of time-lapse images of the stamping
process. In this experiment, FIG. 7A shows an image immediately
prior to etching, FIG. 7B is midway through etching (about 12
minutes) and FIG. 7C when etching is substantially complete (about
50 minutes). Prior to the film etching, a cyclic voltammetry
characterization of the silver sulfide stamp is performed, the
results of which reflect the typical redox potential of Ag/Ag.sup.+
(see FIG. 8). The increase starting at the valley signifies the
onset of redox reactions in the contact surfaces between the silver
film, stamp, and the electrode. The area under the curve reflects
the Gibb's free energy required to propel this etching process.
FIG. 9 shows the change in current over time of the stamping
process for four different electric potential differences (0.2V,
0.4V, 0.6V and 0.8V). The current drop during the first few seconds
reflects generation of a silver concentration gradient in the stamp
that increases the resistance to ionic conduction of the silver
ions.
[0080] These experiments validate the feasibility of transferring a
pattern from a stamping tool to the surface of a metal, metal layer
or a metal film, using electrochemistry. As discussed hereinbelow,
using a programmable pattern generation on a solid ionic conductor
substrate permits direct writing of features with nano scale line
width and micro-meter length. The invention also is for active
growing and dissolution of nanometer structures via controlled
electrical potential application.
EXAMPLE 1
Reprogrammable Patterning of Functional Nanostructures Using
Superionic Conduction
[0081] Reprogrammable and reconfigurable active nanostructures and
processes influence the functional materials and devices to obtain
enhanced energy conversion and chemical sensing. These experiments
address outstanding issues in molecular-scale nanofabrication with
superionic conduction by: (1) Addressing and explaining the
underlying mechanisms of nanoscale charge, mass and energy
transport, and reaction kinetics involved in nanostructure
formation as a result of ionic conduction in solids; (2)
Identifying the factors controlling growth rate and shape fidelity
in the grown structures and exploiting this knowledge to develop a
highly scalable and reprogrammable, in-parallel transfer stamping
process; (3) Exploiting the new capability of programmable and
reconfigurable patterning of nanostructures to actively regulate
ionic transport and electron flow towards enhanced energy
conversion and chemical sensing. The methods disclosed herein
utilize reprogrammable nanopatterning. The fundamental
understanding of nanostructure growth by ionic conduction and the
ability to control it, is further useful for practical design and
manufacturing guidelines for compact and efficient energy storage
and conversion devices.
[0082] Emerging nanotechnology is increasingly focused on the
design and manufacture of nanostructures and nanodevices at scales
that involve a few molecules to exploit capabilities and
functionalities associated with unique physical and chemical
properties identified at these length scales. Qualitatively new
behavior often emerges in nanostructured materials due to
significant confinement and size effects. New modes of transport
for electrical current and/or heat can be obtained when the size of
a nanoscale structure becomes less than the characteristic length
scale for scattering of electrons or phonons (the mean free path).
Similarly the emergence of fundamentally new modes of ionic
transport is predicted in nanostructures. Such optimism is
supported by dramatically enhanced room-temperature ion
conductivity in 1D superlattice systems (Sata et al, 2000) with a
characteristic thickness comparable to space charge layers; This
opens up new routes to electrochemical devices with enhanced energy
conversion and storage density.
[0083] Besides the development of nanoionic devices, such
superionic conduction is useful as the basis of efficient and
cost-effective processes to produce nanostructures and patterns.
Unlike the inefficient and expensive top-down processes and the
low-yield nanoimprint lithography processes, superionic conduction
can be used as the basis of a manufacturing platform that is
efficient, cheap and reprogrammable.
[0084] The invention disclosed herein, enables fast and reversible
growth and dissolution of metallic (including, but not limited to,
silver) nanoclusters, for active and reprogrammable nanopatterning,
based on room-temperature solid ionic (superionic) conductors.
Superionic conductors used to design a fast switch (Terabe et al,
2005) suggests that superionic conduction may be ideally suited for
the development of both nanoscale processes and devices (e.g. see
the switch in FIG. 1 of Terabe et al., 2005). Studies disclosed
herein indicate that both additive and subtractive
nanomanufacturing are possible with the superionic conduction (see,
for example, FIGS. 1, 3 and 10).
[0085] FIG. 10B schematically summarizes use of a pre-patterned
stamp 70 comprising a solid-state ionic conductor 10 to etch metal
20. FIG. 10B illustrates a power supply 35 to energize a cathode 50
and anode 40 to generate an electric field and make recess features
65 on the surface of a metal 20.
[0086] A comprehensive understanding of the superionic conduction
at the nanoscale is useful for formulating and designing
nanostructured materials with tunable and controllable ionic
conductivity and ion storage density at room temperatures.
[0087] Fabrication and experimental characterization methodology
for controllable nanostructure growth/removal with superionic
conduction by direct writing of nanopaterns on the superionic
conductors. We use an electrochemical atomic force microscope
(EC-AFM) to trigger silver growth through ion migration in
Ag.sub.2S to demonstrate that nanoscale line patterns can be
directly written (FIG. 2). We also demonstrate that the patterns
are erasable with a reversed polarity of applied bias (FIG. 3).
Accordingly, controllable silver nanostructures can be achieved via
electrochemical means. To maximize resolution of structures
obtained by the methods disclosed herein, superionic conductor
substrates preferentially have predictable stoichiometry and low
surface roughness. For example, the processes disclosed herein can
use superionic conductor substrates comprising single crystal
.beta.-Ag.sub.2S as a superionic conductor substrate. A wide
variety of superionic conductors can be prepared using
state-of-the-art crystal growth facilities. Pressing, slip casting,
extrusion, and sintering are examples of other methods used to form
polycrystalline or composite superionic conductor substrates.
[0088] Furthermore, understanding and characterizing growth and
removal rates as a function of applied potential, regulated
tunneling current and environmental temperature further assists in
maximizing the resolution and reproducibility of patterns generated
by the processes and devices disclosed herein. Depending on the
process utilized (e.g. writing versus stamping) growth mechanisms,
such as those that occur when growth patterns transition from
controlled cluster growth under the electrode to widespread
spontaneous growth distributed on the surface of the superionic
conductor, vary.
[0089] Many superionic conductors are mixed conductors, conducting
both electrons and ions, so that electronic conduction also plays
an important role in conduction. To enhance the selectivity of
superionic conductors, schemes to limit the electronic current by
forming p-n junctions as known in the art can be utilized.
[0090] Because solid state etching at nanoscale is a relatively
unexplored area, we use the EC-AFM studies to provide insight to
the process mechanisms and limiting factors.
[0091] Mass and charge transport involved in superionic conduction
and the growth/dissolution process. While the field of solid state
ionics has been an area of major scientific and technological
interest in the past, the experimental techniques have primarily
focused on bulk material properties. Only very recently was an
enhanced room-temperature ion conductivity (>4 orders of
magnitude) in 1D layer-by-layer systems reported (Maier, 2000),
indicating the emergence of fundamentally new modes of ionic
transports with a characteristic thickness comparable to space
charge layers. The success of engineering ionic transport in
nanoscale confinement opens up new areas of design and
manufacturing nanoionic structures and devices with improved
efficiency. The benefit of narrowly spaced interfaces that act as
fast pathways for ions or components lies not only in the enhanced
effective conductivity but also in the possibility of rapid bulk
storage resulting from the reduction of the effective diffusion
length.
[0092] Theory and modeling of combined ionic and electronic
transport and growth kinetics at a wide range of dimensional scales
plays a critical role in designing and controlling the growth and
dissolution of metal (including silver) nanostructures with
molecular scale accuracy. Molecular dynamics and embedding
multiscale methods combining quantum-mechanical, atomistic and
continuum theories for electrically-mediated fluid/ion flow in
nanometer channels assist in understanding the fundamental
electrochemical kinetics. Atomic-scale kinetic Monte Carlo methods
for simulating surface shape evolution in chemically reactive
systems, and developing multiscale modeling methods that treat
nanoscale manipulation as a design focus further assist in
maximizing the resolution and reproducibility of pattern generation
methods and devices disclosed herein. Techniques, ranging from ab
initio molecular dynamics, kinetic Monte Carlo, continuum and
multiscale theories further assist in exploring the underlying
fundamental mechanics such as growth, kinetics and transport
properties of silver ions and the combined ion and electron
transport due to the applied electric field. Such modeling, in
combination with empirical data such as those shown in FIG. 11,
provides greater insight into the underlying fundamental
mechanisms.
[0093] Developing reconfigurable and reprogrammable stamping
processes with superionic conductors. The coupling of mass and
charge transport in ionic conduction leads very naturally to the
development of fine resolution etch and deposition processes. The
experiments and models disclosed herein provide insight and control
of the growth process, upon which reprogrammable stamping tool
comprising a superionic conductor is based. FIG. 12A shows a fixed
stamp 70 having an Ag.sub.2S 10 nanostructure combined with limited
process control (e.g. applying an electric field between electrodes
40 and 50, column 55 containing an optional force actuator and
electrode) to successfully generate a transfer patterns by etching
through a silver film 20 with sub-micron features such as scratches
on the stamp surface showing up on the etched substrate, as well as
any number of geometrical features. In an embodiment, an elastic
material 75, such as rubber, assists in maintaining uniform
pressure between the stamp 70 and metal 20.
[0094] Reconfigurable and reprogrammable stamps are particularly
useful when coupled with real-time sensing and growth control of
pattern generation. For example, embedding very precise electronics
into the stamp to estimate growth through changes in conductance
between the stamp and the substrate, as well as control strategies
to overcome the effects of unevenness of stamps by voltage
regulation are two examples where reconfiguragable stamps are
useful. By manipulating the substrate surface by various types of
surface pre-conditioning, the transfer of metal nanostructures onto
a variety of materials is possible, thereby ensuring the process
summarized in FIG. 12 are widely applicable.
[0095] Design of novel active devices using superionic conduction.
Active devices can exploit superionic conduction and be fabricated
by the methods disclosed herein. As shown by FIG. 13, the
superionic nanopatterning technology not only fabricates
nanostructured fuel cells and batteries with improved ionic
conductivity, but also enables novel electrochemically switchable
logic and sensing devices useful in an integrated nanoionic systems
that is self sustainable from the chemical energy stored or
harvested from environment. Here we take the example of an
electrochemically tunable plasmonic switch to show the broad
application of nanoionic devices.
[0096] As the current architectures of high speed nanoelectronics
face challenges of power density and heat management, all optical
computation using nanophotonics may provide an alternative route
towards parallel information processing at high device densities.
The nanopatterning of silver with superionics now offers a
potential for novel active and reconfigurable plasmonic device.
FIG. 14 shows a schematic for an electrochemically tunable
plasmonic switch. The silver nanodots form a plasmonic optical
waveguide and because the shape and dimension of the dots made them
sensitive to the wavelength and polarization, a plasmonic modulator
is realized by dissolving or growing the silver dots at the
junctions. The fast and reconfigurable plasmonic interconnect
enabled by the superionic conduction provides new architectures for
the emerging all-optical computation.
[0097] Characterization of nanoscale superionic conduction and its
exploitation in the development of nanoscale superionic devices
provides an integrated platform for devices that deal with energy
and information. The ability to inexpensively pattern and process
functional materials by the present invention at the nanometer
scale is an important asset in designing new-generation fuel cells
and batteries with integrated systems for sensing and control, and
with increased efficiencies that accrue from the exploitation of
fundamental phenomena of nanometer scale solid state mass transport
and charge separation in energy science.
[0098] Through ionic patterning and switching disclosed herein,
understanding of basic mass transport and solid state chemistry at
nanometer scales is significantly advanced. This, in turn, assists
in optimizing the nanomanufacturing process and tool design,
leading to efficient manufacturing and reduced energy consumption.
Also, the new manufacturing capabilities, which can ultimately lead
to a roll-to-roll type process for nanopatterning, are the basis
for new devices and products in photovoltaic and display
technology.
[0099] Novel processes for generating sub-hundred nanometer
features is presented herein, that integrates and extends the
concepts of nanoimprint lithography and electrochemical
micromachining. Realized by the mass transport property of
solid-state ionic conductors and their dimensional integrity, this
technique provides simplicity and high throughput of single-step
pattern generation while keeping high feature resolution and
reproducibility. Solid-state ionic conductor silver sulfide is
chosen and made into a stamping tool on which calibration features
are defined to verify the lateral resolution capabilities of this
technique. Stamping is achieved under various driving potentials
and sub-hundred-nanometer lateral resolution is obtained. Even
without optimization of the process parameters and environmental
factors, this direct patterning technique shows the potential to
achieve single-step transfer of sub-hundred nanometer feature with
low energy consumption, as well as the flexibility to be integrated
with other nano-fabrication techniques for applications such as
chemical sensors and photonic structures.
EXAMPLE 2
Direct Nanopatterning with Solid Ionic Stamping
[0100] FIG. 15A is an SEM image of an Ag.sub.2S solid state ionic
conductor stamp used for electrochemical stamping and FIG. 15B an
SEM image of the corresponding pattern etched in Ag metal.
[0101] This example discloses an embodiment for generating
sub-hundred nanometer features that integrates and extends the
concepts of nanoimprint lithography and electrochemical
micromachining. Realized by the mass transport property of
solid-state ionic conductors and their dimensional integrity, this
technique provides simplicity and high throughput of single-step
pattern generation while keeping high feature resolution and
reproducibility. In an embodiment, the solid-state ionic conductor
is silver sulfide and is made into a stamping tool on which
calibration features are defined to verify the lateral resolution
capabilities of this technique. Stamping is achieved under various
driving potentials and sub-hundred-nanometer lateral resolution is
obtained. Even without optimization of the process parameters and
environmental factors, this direct patterning technique achieves
single-step transfer of sub-hundred nanometer feature with low
energy consumption, as well as the flexibility to be integrated
with other nano-fabrication techniques for applications such as
chemical sensors and photonic structures.
[0102] Surface micromachining of sub-micron features plays a
substantial role in the fabrication of a wide variety of sensor
devices and microelectromechanical system (MEMS) components. These
techniques realize the generation of such features through either
removing material from substrate, top-down etching, or adding
materials, bottom-up deposition, to build up the desired features.
Among the "top-down" fabrication techniques nanoimprint lithography
followed by dry/wet etching, and electrochemical machining (EM)
provides features with size down to tens of nanometers. Nanoimprint
lithography followed by chemical/physical etching of substrate
provides high feature geometrical and dimensional integrity at the
expanse of multi-step, complex lithography processes that require
stringent process environment control and high-cost equipments.
[0103] The novel patterning technique presented herein extends the
concepts of state-of-the-art nanoimprint lithography and
electrochemical micromachining. The solid ionic stamping
demonstrated in this example exploits the mass-transfer property of
solid state ionic conductors to produce sub-hundred-nanometer
features with high throughput and reproducibility.
[0104] Electrochemical machining that utilizes the local
dissolution of metallic substrate ions and mass transport in the
etching medium by liquid electrolye, can achieve nanometer feature
generation with relatively process simplicity and low cost. The
feature-transfer fidelity, however, degrades as feature size
reduces. As feature approaches the limits where the necessary
replenishment of liquid electrolyte etching medium becomes limited,
features like sharp edges and thin lines lose their geometrical and
dimensional integrity when transferred from machining tool to
substrate surface.
[0105] In an embodiment the present invention uses solid ionic
stamping. The solid ionic stamping presented herein, in contrast to
current electrochemical machining techniques, provides high feature
geometrical and dimensional fidelity in generating the desired
metallic feature using a relatively simple single-step feature
transfer. In addition, the process is low cost while eliminating
the need for sophisticated process equipment while maximizing
feature-transfer fidelity due to the physical property nature of
the etching medium and stamp. The electrochemical stamp using a
solid state ionic conductor stamp of the present invention also
avoids the need for post-treatment of the etching medium used for
metal etching.
[0106] FIG. 16 is a schematic of an embodiment for ionic migration
of silver species in a solid state ionic conductor, silver sulfide.
When subjected to an electric field applied across a silver 20
-silver sulfide 10 interface through anode 50 and cathode 40
attached to them respectively, silver atoms oxidize into mobile
ions 42 and electrons 44. Mobile silver ions 42 move from the
interface through the conduction channels formed in the silver
sulfide bulk 10, toward the cathode 50. Upon receiving electrons 44
when reaching the cathode surface, silver ions reduce back to atoms
43 and deposit on the interface between the cathode 50 and
Ag.sub.2S 10. The oxidation reaction at the interface between metal
20 (e.g. anode) and Ag.sub.2S 10 is used as a tool for surface
micromachining. The advantage of using solid state ionic conductors
is that mass transport only occurs at the surfaces of film metal
anode and solid ionic conductor where physical contact exists,
making it an ideal tool for pattern transfer. In this work, silver
sulfide is synthesized and formed into a tool having a patterned
surface, including nano-scale dimensioins, for use as a stamp 70 to
perform surface micromachining on a silver substrate. The stamp 70
etches metal 20 resulting in a pattern of etched recess feature 65
(FIG. 16C).
[0107] Mass transport coupled with ionic migration in electrolyte
subjected to electrical field, have been used to develop patterning
techniques and devices. A quantized conductance atomic switch that
has been developed wherein silver mobile atoms bridge and open a
tunneling gap between Pt and silver sulfide wires when driven by a
gate potential. Such a switch is reported to be capable of
operating at 1 MHz with low a driving voltage of 0.6V, adding
another nano-scale switch operating at high frequency yet low
energy consumption. With the same ionic mass transport concept,
nanopatterning techniques have been developed to achieve
sub-hundred nanometer line writing and dot deposition with scanning
probe microscopy. These techniques utilize the electric potential
applied across a scanning probe and desired substrate surface and
the migration of metal ions from a solid-state ionic conductor
forming either the substrate or scanning tip to realize the
generation of single line writing or metal dots deposition. The
practicality of this direct pattern writing is limited by the low
throughput and high complexity and cost of the instrumentation
involved. With the aid of a high strength tool material like
Tungsten, the resolution of electrochemical machining has been
pushed to the sub-hundred nanometer regime. The pattern dimension
fidelity and pattern geometry of the transferred feature, however,
is limited by the current density distribution in the liquid-state
electrolyte and its physical properties.
[0108] Lithographic processes followed by chemical/physical etching
of metal have been developed to a point where the cost is minimized
and process standardized to maximize the yield. Those processes
remain expensive, however, as skilled personal and chemical
handling and waste treatment are essential to the operation of the
multi-step processes.
[0109] The present invention described herein extends the
application of mass transport properties of electrolyte to far
beyond the generation of simple geometrical pattern generation with
complex scanning microscopy systems. Instead, the patterns created
are intricate two and three-dimensional patterns in the sub-hundred
nanometer scale within a single-step, high throughput process.
[0110] Silver sulfide synthesis/stamp preparation. An
electrochemical cell is designed to perform the synthesis of silver
sulfide crystals. The cell is composed of a 6 mm-ID quartz tube
with both ends open. In the tube a silver iodide pellet pressed
from powder is placed in contact with a silver pellet on one side
which allows the transport of silver ions from silver across silver
iodide, and the other side exposed to sulfur which allows silver
ions to react with sulfur. A cell potential of 800 mV is then
applied through two electrodes attached to the free surfaces of
silver and silver iodide pellets when the cell reached a reaction
temperature of 360 degrees centigrade. At these reaction conditions
silver ions then migrated through silver iodide layer to cathode
and reacted with sulfur, forming a silver sulfide pellet of a few
millimeters thick. The reaction proceeded until the cell current
dropped to a steady value, indicating the end of reaction, and was
then cooled to room temperature.
[0111] Another method is also employed for silver sulfide
synthesis. A glass tube is filled with sulfur and pressed against a
silver pellet sitting in a glass test tube. The tube is then heated
to 400 degrees centigrade to allow silver-sulfur reaction. The
glass tube pressed against the silver pellet prevents further
growth of the porous silver sulfide layer closer to the silver side
in the formation of silver sulfide layer, and promotes the desired
dense silver sulfide near the sulfur side to further increase
thickness. The synthesized silver sulfide can be as thick as
centimeters, depending on the amount of silver and sulfur
available. The synthesized silver sulfide pellet is then shaped and
patterned with focused ion beam to be used in the subsequent solid
ionic stamping. Calibration features are made such that the
resolution limits are explored. Silver substrate is prepared with a
250 nm-thick silver film deposited with electron beam evaporation
on a 300-.mu.m thick glass cover slip. The silver substrate is
electrically connected to an electrode through physical contact
with a copper electrode. The surface area of the metal substrate
facing the solid state ionic conductor can have any value,
including a range from about 100 .mu.m.sup.2 to about 5
mm.sup.2.
[0112] Silver sulfide stamp characterization. Before patterning
with FIB, the synthesized silver sulfide stamp is characterized
with x-ray diffraction ("XRD") for composition and cyclic
voltametry for electrochemical response. The XRD is conducted on a
Rigaku D-Max system with a scanning range (2-theta) from 0 to 60
degrees and a scan rate of 1.5 degrees per minute. XRD
diffractogram are overlaid and compared with standard peaks from
powder form silver sulfide. The results confirm the composition of
synthesized silver sulfide. The silver sulfide stamp is then
characterized with cyclic voltametry running at 0.5 Hz with a range
from positive 2 volts to negative 2 volts. Characteristic
histeresis confirm the electrochemical behavior of synthesized
silver sulfide.
[0113] Solid state electrochemical etching. Solid-state etching is
performed at room temperature at 1 atmospheric pressure. The
prepared silver sulfide stamp is attached to a platinum electrode
which is fixed to a micro-stage for positioning. On another
micro-stage silver substrate is fixed onto a quartz window with a
platinum electrode attached to it. An optical microscope is built
and placed on the back side of the quartz window for positioning
and process monitoring. Solid ionic stamping is performed by
bringing the stamp in contact with the silver substrate and the
polarity of the electric field is chosen such that silver is the
anode and the Ag.sub.2S side electrode is cathode. Different
electrical potentials ranging from 0.2 V to 0.8 V with an interval
of 0.2 V are applied and current monitored with a Potentialstat.
The processes are also optically monitored with an optical
microscope observing from the back side of the quartz window upon
which the silver substrate resides. The silver film thickness
decreased as stamping proceeded, leading to a continued chromatic
change in the optical image of the film. After stamping, the silver
substrate is then characterized with Atomic Force Microscopy (AFM)
and Scanning Electron Microscope (SEM).
[0114] Solid-state electrochemical etching: Etch kinetics.
[0115] It is known that the ionic conduction of silver sulfide is a
contribution from solubility of silver in silver sulfide. Shown in
FIG. 17 is the cyclic voltametry of the silver sulfide stamp
measured between two platinum electrodes. The two humps on the
ramp-up and ramp-down curves correspond to the increase in overall
conductivity induced by the migration of excessive silver in silver
sulfide, confirming the electrochemical behavior of synthesized
Ag.sub.2S. When a potential field is built up across the silver
sulfide stamp, the migration of the excessive silver in the stamp
along the direction of the field and the polarization effect
resulting from the silver concentration gradient due to silver
migration start to take place. These two effects counteract each
other--as more silver ions become mobile charge carriers and move
from the anode side to the cathode side of the silver sulfide
stamp, the concentration of silver in silver sulfide reduces while
the concentration on the cathode side increases. This concentration
gradient then builds up a polarization effect within the stamp
which acts as a resistive force to the migration of silver ions. As
potential keeps increasing, the polarization effect also keeps
growing to the point where the ionic current is completely blocked,
resulting the current drop on the CV diagram. For this reason, it
is thought that the maximum ionic mass transport efficiency occurs
at the potential where the corresponding current peaks out.
[0116] FIG. 18 shows the current monitored over etching time for
different driving potentials. The currents monitored in this
experimental setup are the combined effects of ionic and electrical
current, due to the mixed-conduction nature of silver sulfide.
Currents for different voltage level follow the same trend--they
reach their highest value within the fist stage of etching, slope
down to lower values in the second stage, and then at the last
stage drop to a steady-low value, indicating the end of etching.
The three-stage behavior can be explained in a similar manner as is
in explaining the CV diagram: During the first and the second
stage, three effects play different roles that contribute to the
overall current output. The increase in silver concentration due to
the dissolved silver from silver substrate into the silver sulfide
stamp increases the ionic current. At the same time, the growing
polarization effect gradually increases, thereby reducing the
mobility of silver ions, contributing an increasing reduction in
the ionic current. As the concentration reaches the solubility
limits of silver in silver sulfide, a steady-state is reached where
further dissolution of silver into silver sulfide from the anode
results in a reduction of silver ion to silver atoms depositing on
the cathode, facilitating the advance of negative stamping. At the
start of the last stage, the depletion of silver on the anode side
results in the reduction and eventually the elimination of ionic
current, leaving the current pure electrical. This is a good
indication of the end of etching.
[0117] Resolution: Depth of features & sidewall angles.
[0118] FIG. 19 is a side by side comparison of the silver sulfide
stamp and the produced silver feature. As seen in FIGS. 19A and 19B
(the stamp) and 19C (the etched substrate), all the geometrical
features are successfully transferred--part of the silver film is
removed through contact with the flat surface area on the silver
sulfide stamp, leaving behind the structures corresponding to the
recess area made on silver sulfide tool. The lateral resolution
achieved is 90 nm on a straight line, calibrated with AFM. The
height of the silver features remains at 250 nm, the silver film
thickness prior to etch, confirming that the bottom surface of the
recess features on silver sulfide is not in contact with the silver
substrate during the process.
[0119] Line widths of the recess feature on the silver sulfide
stamp and the finished silver pattern are calibrated with AFM and
recorded. The generated feature on the etched pattern has a lateral
shrinkage as compared to the designed feature size on the silver
sulfide stamp. The etched feature has a tendency to have a smaller
dimension than that expected from stamp. FIG. 20 shows the lateral
shrinkage for different line widths. For the four driving potential
tested, 0.6V has the lowest lateral shrinkage over the entire size
range while 0.2V and 0.4V have higher lateral dimension reduction.
For the smallest designed line width, 110 nm, the feature comes out
to be 95 nm, a 13% reduction, for 0.6V; whereas the lateral
reductions for 0.2V and 0.4V are 21% and 34% respectively. Without
wishing to be bound to any particular theory, this effect is
believed to be caused by the electrostatic force between stamp
pattern side walls and the resulting silver features, which pulls
silver grains out of the silver feature across the gap that is
formed as stamping proceeds. As the gap increases to the size where
the electrostatic force is small enough to be balanced by the bind
force between silver grains, the remaining silver stays stable.
Generating plots such as those shown in FIG. 20, allows for
compensation in the stamp size lines so as to obtain lines in the
etched material of a given dimension.
[0120] Stamping cycle time. As shown in FIG. 18, the current drops
to a steady lower value when all the excess silver dissolved in
silver sulfide and the silver film in contact with silver sulfide
stamp is depleted, indicating the end of etching process. The etch
rate is calculated at different driving potentials and presented in
FIG. 21. The etch rate ranges from 0.7 nm per second at 0.2V to 2.2
nm per second at 0.8V, comparable to conventional dry etching of
silver. The power consumption of this solid ionic stamping,
however, is orders of magnitude less then those dry etching
techniques.
[0121] Surface roughness and resolution. Also shown in FIG. 21 is
the surface roughness of the resulting etched silver structure for
different driving potentials. The surface roughness is measured by
AFM in 1 micron square. Although the process driving potential does
not have strong effect on the roughness of the etched surface, at
0.4V and 0.6V the resulting silver surface is relatively less
rough.
[0122] Effect of reusing the stamp. Reusing the stamp causes the
features of 50 nanometer lines to collapse. This effect is believed
to be caused by the repeated mechanical contact of the silver
sulfide stamp and silver surface and the force when the two
surfaces are engaged. The force is regulated by setting a fix
position to which the stage controlling the stamp moves in every
run of experiment. Reuse of the stamp does not show strong effects
on the roughness of the stamp; it remains the same after the stamp
has been use for 8000 seconds on actual etching time.
EXAMPLE 3
Electrochemical Stamping
[0123] The invention disclosed herein provides a unique and new
capability to pattern metals with sub-100 nm resolution in a
high-through put stamping process. For example, FIG. 22 shows
arrays of patterns can be etched in metal. Depending on
manufacturing considerations, such a 5.times.5 array may be created
using single step electrochemical stamping, or alternatively, the
metal may be repeatedly stamped to obtain the array. FIG. 23 shows
the stamps of the present invention provide structures having
sub-100 nm resolution. For example, multiple distinct lines or
channels are generated that are separated by 50 nm and the lateral
resolution is 60 nm. In an embodiment, the process is a
solid-state, room temperature process that is highly compatible
with a large variety of process technologies. Although the examples
provided herein utilize silver, different ionic crystals for other
materials, including but not limited to copper, and gold, can be
similarly used, to obtain patterns composed of these other
materials.
Statements Regarding Incorporation by Reference and Variations
[0124] All references cited throughout this application, for
example patent documents including issued or granted patents or
equivalents; patent application publications; and non-patent
literature documents or other source material are hereby
incorporated by reference herein in their entireties, as though
individually incorporated by reference, to the extent each
reference is at least partially not inconsistent with the
disclosure in this application (for example, a reference that is
partially inconsistent is incorporated by reference except for the
partially inconsistent portion of the reference).
[0125] Every formulation or combination of components described or
exemplified herein can be used to practice the invention, unless
otherwise stated.
[0126] Whenever a range is given in the specification, for example,
a temperature range, a size range, a conductivity range, a time
range, or a composition or concentration range, all intermediate
ranges and subranges, as well as all individual values included in
the ranges given are intended to be included in the disclosure. It
will be understood that any subranges or individual values in a
range or subrange that are included in the description herein can
be excluded from the claims herein.
[0127] All patents and publications mentioned in the specification
are indicative of the levels of skill of those skilled in the art
to which the invention pertains. References cited herein are
incorporated by reference herein in their entirety to indicate the
state of the art as of their publication or filing date and it is
intended that this information can be employed herein, if needed,
to exclude specific embodiments that are in the prior art.
[0128] As used herein, "comprising" is synonymous with "including,"
"containing," or "characterized by," and is inclusive or open-ended
and does not exclude additional, unrecited elements or method
steps. As used herein, "consisting of" excludes any element, step,
or ingredient not specified in the claim element. As used herein,
"consisting essentially of" does not exclude materials or steps
that do not materially affect the basic and novel characteristics
of the claim. In each instance herein any of the terms
"comprising", "consisting essentially of" and "consisting of" may
be replaced with either of the other two terms. The invention
illustratively described herein suitably may be practiced in the
absence of any element or elements, limitation or limitations which
is not specifically disclosed herein.
[0129] One of ordinary skill in the art will appreciate that
starting materials, materials, reagents, synthetic methods,
purification methods, analytical methods, assay methods, and
methods other than those specifically exemplified can be employed
in the practice of the invention without resort to undue
experimentation. All art-known functional equivalents, of any such
materials and methods are intended to be included in this
invention. The terms and expressions which have been employed are
used as terms of description and not of limitation, and there is no
intention that in the use of such terms and expressions of
excluding any equivalents of the features shown and described or
portions thereof, but it is recognized that various modifications
are possible within the scope of the invention claimed. Thus, it
should be understood that although the present invention has been
specifically disclosed by preferred embodiments and optional
features, modification and variation of the concepts herein
disclosed may be resorted to by those skilled in the art, and that
such modifications and variations are considered to be within the
scope of this invention as defined by the appended claims.
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