U.S. patent application number 13/210318 was filed with the patent office on 2011-12-08 for apparatus for direct fabrication of nanostructures.
This patent application is currently assigned to MASSACHUSETTS INSTITUTE OF TECHNOLOGY. Invention is credited to Aggelos Bletsas, Brian Hubert, Joseph Jacobson.
Application Number | 20110297084 13/210318 |
Document ID | / |
Family ID | 31498250 |
Filed Date | 2011-12-08 |
United States Patent
Application |
20110297084 |
Kind Code |
A1 |
Hubert; Brian ; et
al. |
December 8, 2011 |
Apparatus for Direct Fabrication of Nanostructures
Abstract
An all-additive apparatus for direct fabrication of
nanometer-scale planar and multilayer structures that performs
"pick-and-place" retrieval and deposition of materials comprises a
tip and a controller and transport mechanism configured for causing
the tip to acquire a transferable material and deposit at least a
portion of the acquired transferable material at a predetermined
location onto a substrate, without the use of a bridging medium, in
order to directly assemble a structure. The tip may be
submillimeter-scale, may comprise a plurality of sub-tips disposed
in a predetermined arrangement, and/or may mechanically vibrate.
Mechanical vibration of the tip may be monitored. The tip may
acquire the transferable material from a reservoir. The assembled
structure may be cured on the substrate.
Inventors: |
Hubert; Brian; (Menlo Park,
CA) ; Jacobson; Joseph; (Newton, MA) ;
Bletsas; Aggelos; (Cambridge, MA) |
Assignee: |
MASSACHUSETTS INSTITUTE OF
TECHNOLOGY
Cambridge
MA
|
Family ID: |
31498250 |
Appl. No.: |
13/210318 |
Filed: |
August 15, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10367616 |
Feb 14, 2003 |
7998528 |
|
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13210318 |
|
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60357006 |
Feb 14, 2002 |
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Current U.S.
Class: |
118/669 ;
977/962 |
Current CPC
Class: |
Y10S 977/849 20130101;
Y10S 977/851 20130101; G03F 7/0002 20130101; Y10S 977/855 20130101;
Y10S 977/86 20130101; B82B 3/00 20130101; Y10S 977/85 20130101 |
Class at
Publication: |
118/669 ;
977/962 |
International
Class: |
B05C 1/00 20060101
B05C001/00; B05C 11/00 20060101 B05C011/00 |
Claims
1. Apparatus comprising: (a) a submillimeter-scale tip; (b) a
controller and, responsive thereto, a transport mechanism,
configured for repeatedly (i) causing the tip to acquire a
transferable material from a reservoir and (ii) pick-and-place
depositing at least a portion of the acquired transferable material
at a predetermined location onto the substrate without a bridging
medium, thereby assembling a structure.
2. The apparatus of claim 1, further comprising means for
facilitating a continuous flow of the transferable material to the
tip from the reservoir.
3. The apparatus of claim 1, wherein the tip comprises a material
selected from the group consisting of a nanotube, a carbon
nanotube, and silicon.
4. The apparatus of claim 1, wherein the tip is mounted on a
bendable cantilever and the controller comprises means for
monitoring and controlling the forces exerted on the tip.
5. The apparatus of claim 4, wherein the controller and transport
mechanism further comprise an actuator and a feedback circuitry for
causing the tip to apply a predetermined amount of force to the
substrate.
6. The apparatus of claim 4, wherein the controller comprises means
for monitoring a force with which the tip deposits the transferable
material in order to determine an amount of the transferable
material deposited.
7. The apparatus of claim 4, wherein the controller comprises means
for monitoring a force with which the tip acquires the transferable
material in order to determine an amount of the transferable
material acquired.
8. The apparatus of claim 7, wherein the controller monitors a
deflection of the cantilever indicative of flooding of the tip and
counteracts the cantilever deflection in response thereto.
9. The apparatus of claim 1, further comprising a curing device for
curing of the transferable material deposited on the substrate.
10. The apparatus of claim 9, wherein the curing device is selected
from the group consisting of: laser sources, ultra-violet light
sources, electron-beam sources, and heat sources.
11. The apparatus of claim 1, wherein the tip comprises a plurality
of sub-tips disposed in a predetermined arrangement.
12. The apparatus of claim 4, further comprising a scanning probe
microscope comprising the nanometer-scale bendable cantilever and
the controller.
13. The apparatus of claim 12, wherein the scanning probe
microscope images the nanometer-scale structure following
deposition thereof
14. The apparatus of claim 1, wherein the tip is configured for
mechanical vibration.
15. Apparatus comprising: (a) a tip comprising a plurality of
sub-tips disposed in a predetermined arrangement; (b) a controller
and, responsive thereto, a transport mechanism configured for
repeatedly (i) causing the tip to acquire a transferable material
from a reservoir and (ii) depositing at least a portion of the
acquired transferable material at a predetermined location onto the
substrate without a bridging medium, thereby assembling a
structure.
16. The apparatus of claim 15, wherein the plurality of sub-tips
simultaneously deposits the transferable material in a
predetermined pattern onto the substrate in a single step.
17. The apparatus of claim 16, wherein the plurality of sub-tips
comprises a stamp having a predetermined pattern of topographical
features.
18. The apparatus of claim 15, wherein the plurality of sub-tips
simultaneously acquires and deposits different transferable
materials.
19. Apparatus comprising: (a) a mechanically vibrating tip; (b) a
controller and, responsive thereto, a transport mechanism for
repeatedly (i) monitoring a shift in a vibration frequency of the
tip, (ii) causing the tip to acquire a transferable material, and
(iii) depositing at least a portion of the acquired transferable
material at a predetermined location directly onto the substrate,
thereby assembling a structure.
20. The apparatus of claim 19, further comprising a reservoir from
which the tip acquires the transferable material.
21. The apparatus of claim 20, further comprising means for
facilitating a continuous flow of the transferable material to the
tip from the reservoir.
22. The apparatus of claim 19, wherein the tip comprises a material
selected from the group consisting of a nanotube, a carbon
nanotube, and silicon.
23. The apparatus of claim 19, wherein tip is mounted on a bendable
cantilever and the controller comprises means for monitoring and
controlling the forces exerted on the tip.
24. The apparatus of claim 23, wherein the controller further
comprises an actuator and a feedback circuitry for causing the tip
to apply a predetermined amount of force to the substrate.
25. The apparatus of claim 24, wherein the controller comprises
means for monitoring a force with which the tip deposits the
transferable material in order to determine an amount of the
transferable material deposited.
26. The apparatus of claim 24, wherein the controller comprises
means for monitoring a force with which the tip acquires the
transferable material in order to determine an amount of the
transferable material acquired.
27. The apparatus of claim 26, wherein the controller monitors a
deflection of the cantilever indicative of flooding of the tip and
counteracts the cantilever deflection in response thereto.
28. The apparatus of claim 19, further comprising a curing device
for curing of the transferable material deposited on the
substrate.
29. The apparatus of claim 28, wherein the curing device is
selected from the group consisting of: laser sources, ultra-violet
light sources, electron-beam sources, and heat sources.
30. The apparatus of claim 19, wherein the tip comprises a
plurality of sub-tips disposed in a predetermined arrangement.
31. The apparatus of claim 24, further comprising a scanning probe
microscope comprising the nanometer-scale bendable cantilever and
the controller.
32. The apparatus of claim 19, wherein the scanning probe
microscope images the nanometer-scale structure following
deposition thereof.
33. The apparatus of claim 19, wherein the tip vibrates when
depositing the acquired transferable material onto the
substrate.
34. The apparatus of claim 19, wherein the tip vibrates when
acquiring the transferable material.
35. The apparatus of claim 19, wherein the controller controls the
descent of tip towards the substrate by monitoring the shift in a
vibration frequency of the tip.
36. The apparatus of claim 19, wherein the controller controls the
amount of the transferable material acquired by the tip by
monitoring the shift in a vibration frequency of the tip.
37. The apparatus of claim 19, wherein the controller controls the
amount of the transferable material deposited onto the substrate by
the tip by monitoring the shift in a vibration frequency of the
tip.
Description
RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. patent
application Ser. No. 10/367,616, filed Feb. 14, 2003, now U.S. Pat.
No. 7,998,528, which claims priority to and the benefits of U.S.
Prov. App. Ser. No. 60/357,006, filed on Feb. 14, 2002, the entire
disclosures of which are hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention generally relates to the field of
microfabrication, in particular, to direct fabrication and imaging
of nanometer-scale structures using a scanning probe microscope,
e.g. an atomic force microscope.
BACKGROUND
[0003] Nanotechnologies promise to bring about the advent of very
small, yet important electronic and biological devices with
features that are only a few tens of nanometers across. A variety
of nanometer-scale ("nano") materials, such as carbon nanotubes,
nanoparticles, and molecular memories are being developed. However,
improvements in the handling and patterning of these nanomaterials
are necessary before they can be cost-effectively incorporated into
useful nanodevices such as, for example, single-electron
transistors, high-density gene chips, and terabyte-scale memory
systems. These devices require new fabrication and patterning
techniques that far exceed resolution limitations of known
processing techniques.
[0004] For example, known lithographic methods that are at the
heart of modern microfabrication, nanotechnology, and molecular
electronics often rely on patterning a resistive film, followed by
chemical etching of the substrate. A variety of such subtractive
printing techniques employ scanning probe instruments, electron
beams, or molecular beams to pattern substrates using
self-assembling monolayers and other organic materials to form
sacrificial resistive layers. Known microfabrication techniques
such as photolithography, microcontact printing, micromachining,
and microwriting can produce patterns as small as 100 nm, but the
production of sub-100 nm structures still poses a challenge.
[0005] Also, many nanomaterials containing discrete components,
e.g. nanotubes, must ordinarily be manipulated and directly
assembled onto a surface without the assistance of resists, masks,
or etching steps. Furthermore, many organic materials that could be
useful in nanoscale devices, such as DNA and proteins, are easily
damaged and, thus, are difficult to pattern to form very small
structures. Thus, new methods are needed to address the challenge
of patterning and constructing useful nanoscale devices.
[0006] Since its inception, scanning probe microscopy has proven to
be a useful tool for high-resolution imaging of nanoscale
structures. The scanning probe typically includes a cantilever made
of silicon having a length of about 200 .mu.m. The cantilever has a
sharp tip at its end with a radius of curvature generally below 10
nanometers. Depending on the imaging mode used, topological
features as fine as individual atoms can be resolved.
[0007] More recently, it has been shown that the tip of a scanning
probe microscope, such as an atomic force microscope ("AFM"), may
be useful for the direct assembly of nanostructures. The tip can be
very sharp, with only a few atoms located at its apex. A number of
techniques that use an AFM tip to push very small objects,
including atoms, nanoparticles, and nanotubes across a surface to
form simple patterns, have been developed. However, the pushing
operations are very complex, and construction of useful structures
is indirect and often prohibitively tedious.
[0008] Another process, known as "dip pen nanolithography" ("DPN"),
uses an AFM tip to deposit a restricted set of organic molecules
onto carefully chosen substrates. Generally, DPN is a
nanolithography technique by which molecules are directly
transported to a substrate. DPN utilizes a solid substrate as the
"paper" and an AFM tip (or a near-field scanning optical microscope
tip) as the "pen." The tip is coated with a patterning compound
(the "ink"), and the coated tip is used to apply the patterning
compound to the substrate to produce a desired pattern. The DPN
delivery mechanism involves the formation of an adsorbed water
meniscus around the tip to transfer the ink molecules to the
substrate, and the control of the movement of the patterning
molecules to the surfaces on which they are deposited by a driving
force to form self-assembling monolayers.
[0009] Problems that arise with DPN technology stem from the
dependence of this technique on the liquid meniscus and chemical
affinity of the patterning material to the substrate. For example,
the lateral width of the line written by the "pen" using DPN is
limited by the width of the meniscus formed. The meniscus is
subject to variations in the relative humidity as well as chemical
interactions between the solvent and the substrate. The size of the
meniscus may also affect the rate of the transport of the
patterning compound to the substrate. Solubility characteristics of
the "ink" molecules in a given solvent can create difficulty in
establishing a desired line width and a suitable loading
concentration of the ink in the solvent. Furthermore, surface
tension characteristics of different solvents can lead to drip or
rapid flow from the pen resulting in problems with precise control
of the ink application under some circumstances. Finally, special
substrate-liquid interactions and self-assembling chemistries may
be necessary to promote the adhesion of the molecules to the
substrate, limiting the kinds of materials that can be patterned in
this fashion.
[0010] Thus, there remains an unresolved need in the art to enable
rapid and direct patterning of arbitrary materials onto arbitrary
substrates with nanoscale resolutions.
SUMMARY
[0011] It is an object of the present invention to achieve direct
deposition and patterning of nanoplanar and multi-structures at the
desired precise locations.
[0012] It is another object of the present invention to enable
patterning a wide variety of materials having useful electrical,
chemical, mechanical, and biological properties onto a wide variety
of substrates.
[0013] It is yet another object of the present invention to provide
an efficient and inexpensive method of fabricating nanostructures
that does not require tedious pushing or pulling of particles or
other nanometer-sized objects along the surface of a substrate; and
does not require special self-assembling chemistries or
liquid-substrate interactions, such as water vapor-initiated
menisci, to facilitate the transfer of molecules from the tip to
the substrate.
[0014] Accordingly, a high-precision nanoprobe-assisted deposition
process, capable of directly assembling planar and multi-layer
nanostructures by incrementally building them from materials in a
liquid or soft-solid phase, is disclosed herein. Also disclosed
herein is an apparatus implementing such process.
[0015] A key aspect of the present invention involves direct
assembly of planar and multi-layer structures with nanogeometries
by a discrete "pick-and-place" technique using a sharp tip mounted
on a bendable cantilever, for example, a tip of a scanning proble
microscope such as an AFM. The nanoassembly method of the invention
facilitates high-resolution direct fabrication of arbitrary
materials, many of which are not amenable to deposition using
current probe-based patterning, DPN, or conventional lithography
methods. Applications of the nanoassembly system of the invention
include, but are not limited to, fabrication of ultra-density gene
chips, high-capacity magnetic disk drives, and single electron
transistors.
[0016] Unlike probe-based nanomanipulation techniques known in the
art that push atoms or nanoparticles around on a surface, the
nanoassembly method of the invention enables true "pick-and-place"
retrieval and deposition of materials with a wide range of
electrical, chemical, and mechanical properties. The AFM-assisted
nanoassembly method of the invention is an all-additive process
that substantially eliminates any waste. Exceedingly small
quantities of material, a few thousand atoms at a time, can be
picked up by an AFM tip from a reservoir and are then assembled at
a designated construction site on the substrate to fabricate
nanostructures without subsequent application of caustic chemicals,
etchants, and other effluents that are typically used in known
microfabrication methods.
[0017] Further, unlike known DPN methods, the present invention
provides for deposition of various materials, e.g. metal
nanoparticles, polymers, inks, solvents, organics, semiconductor
nanoparticles and dielectric nanoparticles onto a variety of
substrates without formation of an adsorbed substrate-material or
substrate-tip bridging medium, such as a liquid meniscus of water
adsorbed from humid air at the interface between the tip and the
substrate. In accordance with the present invention, materials can
be assembled using reservoirs containing liquids, soft solids, or
collections of discrete nano-scale objects, such as nanoparticles,
that have no chemical affinity to the substrate.
[0018] Throughout the following description, the term "ink" is used
to generally refer to the material being deposited by the
nanoassembly system to form nanoassembled structures. It should be
understood that the term "ink" is used to refer to materials that
are in either a liquid phase or solid phase or some phase in
between (such as a gel or slurry). The term "ink" is also used to
refer to a collection of solid discrete objects such as spheres,
balls, nanoparticles, nanocrystals, nanotubes, nanorods, cubes, and
tetrahedrons. These solid discrete objects may or may not be
entrained in a gas-phase or liquid-phase fluid. The ink materials
and nanoparticles, once assembled by the methods of the invention
onto a substrate, are stable and can be used as the foundation of
multi-layer nanostructures.
[0019] The invention makes use of a very sharp tip to transfer
material. The tip may (but need not be) attached to a flexible
medium such as a cantilever or spring, and may (but need not) be
part of an AFM. In embodiments in which the tip is mounted to a
cantilever, the terms "tip" and "cantilever" are used
interchangeably. It is the tip and the manner in which it is used,
rather than the specific mechanism by which it is operated or
mounted, that is important to the invention.
[0020] According to embodiments of the invention, the tip is
mounted on a bendable cantilever, for example, a cantilever of a
scanning probe microscope, such as an AFM, and operates as a
manipulator of both liquid and solid materials at nanometer scales.
The tip may be controlled as part of the AFM feedback loop
including the following components: a nano-scale tip mounted on a
bendable cantilever; a sensor that monitors the degree of bending
of the cantilever and the frequency of its oscillation (and thereby
monitors the forces exerted onto the tip); and an actuator and
feedback circuitry that cause the tip to deliver a specified level
of force, impact velocity and/or impulse to the substrate. The
dimensions of deposited nanostructures can be controlled by
adjusting, at least (a) the rate of vibration of the tip; (b) the
sharpness of the tip; (c) the viscosity, phase, and material
properties of the ink; and (d) pressure and force impulse applied
by the ink-laden tip to the substrate. For example, with respect to
adjusting the viscosity of the ink, [0021] the smallest depositions
are achieved when using very non-viscous fluids or near-hard solids
that are nearly fully solidified; [0022] larger depositions are
obtained from more viscous fluids or soft solids; and [0023] the
largest depositions are achieved using very viscous liquid-phase
materials.
[0024] The resolution of the resulting nanostructures may approach
1 million dots per linear inch (1 trillion dots per square inch).
Deposited volumes can be precisely controlled to span 10 orders of
magnitude, from 10.sup.-24 to 10.sup.-14 liters. The nanostructures
assembled using the method of the invention may have line widths of
less than approximately 100 nm, with the smallest discrete circular
features being less than 32 nm across. The method of the invention
facilitates creating structures with height-to-width aspect ratios
of better than 1-to-2 and provides improved control over line width
and deposition rate, while being relatively insensitive to
fluctuations in ambient conditions, such as temperature, humidity,
atmospheric conditions, vibration, and thermal drift. A dot of
material can be discretely deposited onto the substrate at rates
greater than 1 dot per second.
[0025] In general, in one aspect, the invention features a
nanoassembly apparatus that includes a nanometer-scale bendable
cantilever having a tip mounted thereon; a controller; and a
transport mechanism. The transport mechanism, responsive to the
controller, causes the tip to discretely acquire a transferable
material; scan the tip over a substrate; and deposit at least a
portion of the acquired transferable material at a predetermined
location directly onto the substrate without a bridging medium,
thereby assembling a nanostructure on the substrate.
[0026] In one embodiment, the nanoassembly apparatus also includes
one or more reservoirs from which the tip acquires the transferable
material. Optionally, the transferable material does not chemically
bond to the substrate upon deposition thereon. The nanoassembly
apparatus may also include means for facilitating a continuous flow
of the transferable material to the tip from the reservoir. In yet
another embodiment, the temperature of the reservoir is controlled
by the apparatus.
[0027] In some embodiments, the tip of the nanoassembly apparatus
is a nanotube, for example, a carbon nanotube. In other
embodiments, the tip is made of silicon.
[0028] The controller of the nanoassembly apparatus may include
means for monitoring and controlling the forces exerted on the tip.
In one embodiment, the controller also includes an actuator and
feedback circuitry for causing the tip to apply a predetermined
amount of force to the substrate. In another embodiment, the
controller also includes means for monitoring a force with which
the tip deposits the transferable material in order to determine an
amount of the transferable material deposited. In yet another
embodiment, the controller includes means for monitoring a force
with which the tip acquires the transferable material in order to
determine an amount of the transferable material acquired. For
example, the controller monitors a deflection of the cantilever
indicative of flooding of the tip and counteracts the cantilever
deflection in response thereto. In yet another embodiment, the
apparatus includes means for detecting flooding of the tip and
counteracts this condition by adjusting the position of the tip in
response thereto.
[0029] In one embodiment, the nanoassembly apparatus includes a
curing device for curing of the transferable material deposited on
the substrate, for example, a laser source, an ultra-violet light
source, an electron-beam source, a heat source, an infra-red light
source, an electric current source, or an electric voltage source.
In one aspect of the invention, the tip comprises the curing
device.
[0030] In some embodiments, the tip of the nanoassembly apparatus
includes a plurality of sub-tips disposed in a predetermined
arrangement. The plurality of sub-tips may simultaneously deposit
the transferable material in a predetermined pattern onto the
substrate in a single step, as well as simultaneously or
independently acquire and deposit different transferable
materials.
[0031] In some embodiments, the bendable cantilever and the
controller are part of a scanning probe microscope, for example, an
atomic force microscope. In one embodiment, the scanning probe
microscope of the nanoassembly apparatus images the nanostructure
following the deposition thereof. In yet another embodiment, the
tip images or detects the structure before, during or after
deposition of the transferable material.
[0032] The tip may be configured for vibration. In one embodiment,
the tip vibrates when depositing the acquired transferable material
onto the substrate. In another embodiment, the tip vibrates when
acquiring the transferable material.
[0033] In some embodiments, the controller monitors the shift in a
vibration frequency and/or vibration amplitude of the tip.
Optionally, the controller controls the descent of the tip towards
the substrate, the amount of the transferable material acquired by
the tip, and/or the amount of the transferable material deposited
onto the substrate by the tip by monitoring the shift in a
vibration frequency and/or vibration amplitude of the tip. For
example, monitoring the vibration amplitude facilitates control of
the tip's descent and/or the amount of transferable material
acquired by (and/or deposited by) the tip.
[0034] In general, in another aspect, the invention features a
method for nanoassembly that includes the steps of providing a tip
mounted on a bendable cantilever, causing the tip to discretely
acquire a first transferable material, scanning the tip over a
substrate, and operating the tip to deposit at least a portion of
the acquired first transferable material at a predetermined
location directly onto the substrate without a bridging medium. The
method further includes repeating the latter three steps to create
a nanostructure using the first transferable material. A resulting
nanostructure may be two-dimensional, or three-dimensional, with at
least some of the transferable material being deposited onto
previously deposited transferable material.
[0035] In one embodiment, the method of the invention also includes
facilitating a continuous flow of the transferable material to the
tip from the reservoir. In another embodiment, the transferable
material does not chemically bond to the substrate upon deposition
thereon.
[0036] The method for nanoassembly may also include monitoring and
controlling the forces exerted on the tip. In one embodiment, the
method includes causing the tip to apply a predetermined amount of
force to the substrate. In another embodiment, the method includes
monitoring a force with which the tip deposits the transferable
material onto the substrate in order to determine an amount of the
transferable material deposited. In yet another embodiment, the
method includes monitoring a force with which the tip acquires the
transferable material in order to determine an amount of the
transferable material acquired. Also, the method may include
monitoring a deflection of the cantilever indicative of flooding of
the tip and counteracting the cantilever deflection in response
thereto. Also the method may include detecting flooding of the tip
and counteracting this condition by adjusting the position of the
tip in response thereto. The method for nanoassembly may optionally
include the step of thermal curing of the deposited transferable
material on the substrate.
[0037] In some embodiments, the tip includes a plurality of
sub-tips disposed in a predetermined arrangement, and at least a
portion of the transferable material is deposited in a
predetermined pattern in a single step by the plurality of
sub-tips.
[0038] In one embodiment, the acquiring step also includes causing
the tip to discretely acquire a second transferable material
simultaneously with the first transferable material. The depositing
step may include operating the tip to deposit at least a portion of
the acquired second transferable material simultaneously with at
least a portion of the acquired first transferable material. In yet
another embodiment, the tip acquires a second transferable material
before, during, or after the tip acquires a first transferable
material. In yet another embodiment, the tip deposits a second
transferable material before, during or after the tip deposits a
first transferable material. At least a portion of the second
transferable material may be deposited so as to overlap at least a
portion of the first deposited transferable material.
[0039] In some embodiments, the method for nanoassembly includes
the step of causing the tip to vibrate. In one version of this
embodiment, the tip vibrates when depositing the acquired
transferable material onto the substrate. In another version, the
tip vibrates when discretely acquiring the transferable
material.
[0040] In one embodiment, the method for nanoassembly includes the
step of causing momentary contact between the tip and the substrate
to deposit a dot of the transferable material. In another
embodiment, the method includes translating the tip along the
substrate to deposit a line of the transferable material. The line
of the transferable material may have a width of less than
approximately 100 nm, for example, approximately 17 nm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] In the drawings, like reference characters generally refer
to the same parts throughout the different views. The drawings are
not necessarily to scale, emphasis instead generally being placed
upon illustrating the principles of the invention.
[0042] FIG. 1 schematically illustrates the nanoassembly apparatus
according to an illustrative embodiment of the invention;
[0043] FIG. 2 depicts a vibration isolation system, which may be
employed with the nanoassembly apparatus of FIG. 1;
[0044] FIG. 3A depicts the steps of a method for nanoassembly of
dotted patterns according to an illustrative embodiment of the
invention;
[0045] FIG. 3B depicts the steps of a method for nanoassembly of
line patterns according to an illustrative embodiment of the
invention;
[0046] FIG. 3C depicts a "flooding compensation" technique, which
may be employed with the methods for nanoassembly of FIGS. 3A-3B
according to another illustrative embodiment of the invention;
[0047] FIG. 4 depicts the steps of a solvent-assisted method for
nanoassembly according to an illustrative embodiment of the
invention;
[0048] FIG. 5 depicts images of sample nanostructures deposited
using the "grayscale" method according to an illustrative
embodiment of the invention;
[0049] FIG. 6 schematically illustrates a nanoassembly apparatus
including a laser curing device according to another illustrative
embodiment of the invention; and
[0050] FIG. 7 depicts the steps of a method for nanoassembly using
the nanoassembly apparatus shown in FIG. 6.
DETAILED DESCRIPTION
[0051] The nanoassembly method of the invention provides for the
deposition of planar, multi-layer, and three-dimensional
nanostructures. In particular, the nanoassembly system can be used
for the fabrication of a number of basic canonical structures
including lines, dots, and columns. These canonical structures can
be combined and adjacently deposited laterally and/or vertically to
form complicated and useful structures, devices, and patterns.
[0052] Referring to FIG. 1, in one embodiment, a nanoassembly
apparatus 100 in accordance with the invention includes a
controller 105 that operates an AFM head 110 in three dimensions
over the surface of a substrate S disposed on top of an adjustable
substrate positioning stage 111. As illustrated in FIG. 1, the
surface of substrate S extends in the (x,y) plane, while movement
toward and away from the surface occurs along the z axis. A
transport mechanism may execute movement along the three axes using
a series of independently operable piezo elements, which are united
into a single tube 112. AFM head 110 comprises a bendable
cantilever 115, which terminates in a tip 120. In one version of
this embodiment, tip 120 is secured directly to the underside of
AFM head 110 by way of a magnetic plate angled at a nominal 4.5
degrees relative to substrate S. The end of the tip may be
submicron in scale, may be less than 10 .mu.m across, or may be
less than 100 .mu.m across. A piezoelectric oscillator 125, itself
operated by a frequency-synthesizer module 127 of controller 105,
deflects cantilever 115 as indicated by the arrow as tip 120 passes
over the substrate S. Alternatively, oscillatory deflection of
cantilever 115 may be provided by one or more piezo elements
defining or within tube 112. The instantaneous degree of cantilever
deflection is monitored by a conventional optical arrangement
comprising a laser 130, a split-photodiode detector 135, and a
detector circuit 140. The output of detector 140 is fed back to
controller 105.
[0053] The system 100 also includes a data-handling circuit 150
that orchestrates nanoassembly operations and facilitates
communication with standard or non-standard computer architectures.
An interface module 155 sends commands to controller 105, causing
tip 120 to be brought adjacent a desired point on substrate S. A
data cache 160 directs transfer of data about the target structure
during nanoassembly, and stores acquired image data in read mode.
The data, in turn, is received from or sent to an operator equipped
with a computer 180 by means of an input/output module 170. In
response to the operator's input, computer 180 through interface
155 causes the controller 127 to direct appropriate movement and
operation of AFM head 110. A source of material 190 to be deposited
onto substrate S, for example an ink reservoir, is disposed in
close proximity to the deposition area of substrate S. The ink
reservoir can be located adjacent to, on, or inside tip 120 (or
cantilever 115), and the system may include conventional
temperature-regulation circuitry to maintain the reservoir at a
desired temperature. The nanoassembly process proceeds under
computer control to dip tip 120 into reservoir 190 of a soft solid
or liquid ink (unless reservoir 190 is located on or inside tip
120), to then translate the tip to the deposition site, and to then
quickly lower tip 120 onto substrate S surface to cause deposition
of the ink. In one embodiment of the invention, the nanoassembly
apparatus 100 is based on an EXPLORER scanning probe microscope
available from Thermomicroscopes of Sunnyvale, Calif., with
NSC12/W2C model tips coated with a conductive layer of tungsten
carbide available from MikroMasch of Moscow, Russia.
[0054] Cantilever 115 of the invention can operate in either of two
modes, namely, contact mode and tapping mode (sometimes referred to
as "non-contact" mode). In a contact mode, cantilever 115 is not
actively deflected. Instead, the tip 120 contacts substrate S (or a
thin layer of molecules adsorbed thereon) as it is scanned over the
surface. Controller 105 moves AFM 110 head along the z axis in
response to the detector signal in order to maintain a constant
cantilever deflection as tip 120 is scanned over the changing
surface topography. By virtue of this feedback loop, the force
between tip 120 and the surface remains constant. To obtain an
image of the topography of a surface, the changing z-axis position
is recorded as the head is scanned over the surface.
[0055] In a "tapping" mode, the form of operation preferred herein,
cantilever 115 is oscillated at or near its resonance frequency
with an amplitude ranging, typically, from 1 nm to 100 nm. For
example, cantilever 115 may be 250 .mu.m in length and composed of
silicon, with a resonance frequency of 30-300 kHz; such elements
are available from Digital Instruments, Santa Barbara, Calif. Tip
120 lightly "taps" on the surface of substrate S during scanning,
contacting (or nearly contacting) the surface at the bottom of its
oscillation excursion. The feedback loop comprising the detector
135, the detector circuit 140, and the controller electronics 105
maintains a constant oscillation amplitude by raising or lowering
AFM head 110 along the z axis to maintain a "setpoint" amplitude
and/or frequency.
[0056] In some embodiments, the tip that is caused to vibrate with
frequencies up to several hundreds of kilohertz is used to both
assemble and then immediately image the deposited nanostructures.
Due to the vibrating action, the same tip that assembles the
nanostructures can go back and scan those nanostructures only
moments later, without unintentionally depositing more material
onto the substrate. In such embodiments, the operator is able to
visualize the nanostructures as they are being constructed in
situ.
[0057] The vibrating mode also provides the piezo actuators of tube
112 controlling the descent of tip 120 (based, for example, on the
vibration amplitude) with advance warning of the approaching
surface S. The actuators can then compensate to dramatically reduce
the impact forces delivered by tip 120 to the substrate S, which
enables the deposition of very high-resolution patterns. Moreover,
monitoring the shift in the vibration frequency of tip 120
facilitates accurate real-time measuring of the mass of material
gained and lost by tip 120 during each step of the pick-and-place
process. Based on such measurements, the process parameters can be
adjusted in the middle of a run to achieve the optimum deposition
conditions for a range of materials.
[0058] To improve accuracy of the microfabrication and imaging of
the deposited nanostructure, it is desirable to provide vibration
isolation to nanoassembly apparatus 100. In one embodiment of the
invention, the vibration isolation may be provided by a
conventional air-levitated isolation table, in which the air is
supplied from a pressurized tank or compressor through a regulator.
Referring to FIG. 2, in another embodiment of the invention, an
elastic (or bungee) cord-supported isolation system 220 provides
vibration dampening to the nanoassembly apparatus 100 while
maintaining a small physical footprint. The isolation system 220
includes a heavy metal plate 230 hanging from a support 240 by way
of two pairs of criss-crossed elastic cords 250. Metal plate 230
serves as the baseplate supporting nanoassembly apparatus 100. To
further restrict movement and improve motion damping, the bottom of
plate 230 is connected to (but still suspended above) a metal plate
260 by two pairs of criss-crossed elastic cords 270. In one version
of this embodiment, plate 260 is heavier than plate 230. Other
configurations of vibration isolation system using one or more
plates supported by elastic cord arrangements, however, can also be
used without deviating from the scope of the invention.
[0059] In yet another embodiment of the vibration isolation system,
nanoassembly apparatus is placed on top of a multi-layer stack of
damping materials, for example, two layers of commonly available
plastic bubble-wrap placed on top of a single layer of one-half
inch thick carpeting.
[0060] Some liquid-phase ink materials used for nanoassembly may
quickly evaporate. Because viscosity fluctuations of the ink
material may substantially affect the parameters of the deposited
structure, it may be desirable to prevent any phase transformations
during deposition. Therefore, it is useful to control the
temperature and atmospheric conditions under which the nanoassembly
apparatus operates. Still referring to FIG. 2, in one embodiment,
nanoassembly apparatus 100 is enclosed in an environmental chamber
280, which preferably is transparent to enable visualization, for
controlling atmospheric conditions and particle count. In this
embodiment of the invention, the nanoassembly process may take
place in custom atmospheres, for example, in argon, helium,
nitrogen, oxygen, or neon. An in-situ temperature control device
290, for example, one or more single- or double-stack peltiers
attached to substrate positioning stage 111 using thermally
conducting paste, is used to perform two functions: (1) cool the
substrate so that the evaporation rate of the ink in the ink
reservoir is reduced, thereby extending the length of time
available for nanostructure fabrication using liquid phase inks;
and (2) heat the substrate for thermal curing of deposited
nanostructures. In one embodiment, when substrate S is being cooled
by the peltier or other temperature-controlling device 290,
vaporless atmosphere is maintained in the environmental chamber 280
to prevent condensation of moisture onto the substrate. In one
version of this embodiment, dehumidification of the area
surrounding substrate S is achieved by supplying pressurized argon,
nitrogen, or other suitable gas into enclosure 280.
[0061] The methods of the invention can be used to assemble
materials with a wide range of varying properties including
nanoparticles (e.g., conductor or semiconductor nanoparticles) with
or without capping groups, microparticles, polymers (e.g.,
nonconductor, conductor or semiconductor particles), ceramics, ink
compositions (e.g., containing nanoparticles and/or nanotubes),
gels, oxides, metals, inorganics, solvents, organics, etchants,
plating solutions, catalysts, light-curable or light-crosslinkable
materials, resists, biological compounds such as genetic and
proteomic materials, light-emitting materials such as LEDs, OLEDs
and light-emitting polymers, and inorganics (hereafter,
collectively, "transferable materials"). Essentially any type and
phase of the ink material that can become temporarily adhered to
the tip, and can then also become disassociated from the tip
thereby allowing the tip to acquire and then deposit the material
can be used. In one embodiment, gold and silver nanoparticle inks,
diluted to reduce viscosity with alpha-terpineol to metal
concentration of about 5-40% by weight, are used. Other inks can
also be used, for example, a silicone-based elastomer pre-cursor,
such as DOW SYLGARD 184, available from Dow Chemical of Midland,
Mich.; a UV-curable clear adhesive, such as NORLAND optical
adhesive #72, available from Norland Products, Inc, of Cranbury,
N.J.; a photoresist diluted with alpha-terpineol, such as SHIPLEY
SPR 3012 or AZ1518, available from Shipley Company LLC of
Marlborough, Mass. The tip of a thin metal wire can be used to form
ink pools as material reservoirs on a suitable substrate, such as a
silicon wafer or a glass slide. To reduce contact angle and reduce
undesirable "flooding" of a cantilever tip upon contact with the
ink, in one embodiment, the ink-laden wire is dragged across to the
substrate to form an elongated oval-shaped pool.
[0062] Nanoassembly deposition can be conducted on a variety of
substrates, including glass, quartz, plastics, polyimide, Kapton,
silicon, and metal foil. In one embodiment, the nanostructure is
deposited onto a silicon wafer.
[0063] Operation of the nanoassembly system of the invention
involves a large number of control parameters and variables. In one
embodiment, the invention utilizes an expert software system that
requests desired fabrication parameters from the user (such as line
width, dot size, viscosity of the ink materials, elapsed drying
time of the ink in the ink reservoir, etc.) and automatically
generates script templates for the computer commands that control
many aspects of the nanoassembly process including tip position and
velocity, tip dwell time, and impact forces used to pick up and
deposit ink materials. Such software simplifies the process of
writing new control code to accommodate some change in nanoassembly
fabrication parameters or constraints. The functions implemented by
such structure are described below.
[0064] Referring to FIG. 3A, in one embodiment, a method for
nanoassembly of dotted patterns using liquid or soft-solid phase
inks begins with dipping the tip into a reservoir of ink with the
cantilever operating in either contact mode or tapping mode (STEP
310). The method further includes withdrawing the tip from the
reservoir with a small quantity of ink material adhering to the tip
(STEP 315). Then, the tip is transported, preferably at a high rate
of speed, for example 1 mm/s to 1 m/s at a height of several .mu.m
above the surface of the substrate to the deposition site (STEP
320). Following arrival at the deposition site, the tip is lowered
and comes into contact with the substrate (STEP 322), thereby
depositing the adhered ink material in the form of a dot of ink
(STEP 325). Generally, no externally supplied bridging medium, such
as a humidity-initiated water meniscus, is required between the tip
and the substrate to facilitate deposition of the adhered ink
material. After the dot is deposited, the tip is immediately raised
from the surface (STEP 330). The method concludes with high-speed
levitated transportation of the tip back to the ink reservoir (STEP
335). The cycle is repeated at a high frequency once for every dot
that is deposited. In one version of this embodiment, the method is
performed at a frequency of at least one cycle per second.
[0065] Referring to FIG. 3B, in another embodiment, a method for
nanoassembly of line patterns using liquid or soft-solid phase inks
includes steps 310-322 described above. Following the arrival at
the deposition site, the tip is lowered until it comes into contact
with the substrate, and is subsequently dragged along the substrate
causing the tip to leave behind a trail of ink. When using
liquid-phase inks, the possibility of deposition of a large
undesirable "bulbous" structure at the beginning of the line is
typically minimized by using a tip operating under tapping mode
instead of contact mode, because the vibration of the tip in
tapping mode prevents the ink on the tip from "flooding" the
substrate (STEP 340). After the line is deposited, the tip is
immediately raised from the surface (STEP 345). The method
concludes with high-speed levitated transportation of the tip back
to the ink reservoir (STEP 350).
[0066] The nanoassembly system can achieve high-resolution
structures by controlling a number of variables and parameters. For
example, typically, the size of deposited dots and lines is
directly proportional to the sharpness of the tip of the cantilever
and the force applied by the tip to the substrate at the point when
the dot is deposited, and inversely proportional to the viscosity
of the ink. Also, when the tip is dipped into the ink reservoir and
controlled so that the tip engages the ink in the reservoir at or
near the damped resonance frequency of the cantilever-reservoir
system, a cantilever operating in a tapping mode typically extracts
a smaller volume of ink from the reservoir compared to a cantilever
operating in a contact mode, thereby enabling subsequent deposition
of smaller dots onto the substrate. Furthermore, a cantilever
operating in a tapping mode when depositing the dot upon the
substrate is capable of providing an advance warning of the
approaching surface as the tip descends to deposit a dot, thereby
reducing the tip-to-surface impact force and enabling the
deposition of smaller dots as compared to the cantilever operating
in a contact mode.
[0067] In some embodiments, when using liquid-phase materials that
are composed of solid phases (such as organically capped
nanoparticles) within a liquid solvent or carrier, it is desirable
to control the viscosity of the ink. Specifically, the viscosity of
the ink can be reduced by adding more solvent to the reservoir;
maintained by cooling the material reservoir to hinder solvent
evaporation; or increased by heating the material reservoir to
enhance solvent evaporation and partial thermal curing. Due to
evaporation effects, the viscosity of the material attached to the
tip may change while the tip is transported from the reservoir to
the deposition site.
[0068] Referring to FIG. 3C, in one embodiment of a method for
nanoassembly using liquid- phase inks, a "flooding compensation"
technique can be employed to prevent undesirable overloading of the
tip with ink that occurs when the tip is lowered into the ink
reservoir and the ink rapidly traverses the entire length of the
cantilever, thereby oversupplying the deposition zone on the
substrate under or adjacent the cantilever with a layer of ink.
This technique includes the following steps: [0069] (1) the tip is
lowered into the ink reservoir (STEP 360) until it contacts the
surface of the liquid-phase ink in the ink reservoir (STEP 365);
[0070] (2) as liquid ink begins to "flood" or rush onto the tip
from the ink reservoir, the feedback circuit of the piezo actuator
of the z axis ("vertical motion actuator") detects the deflection
of the cantilever (when using contact mode) and/or the dramatic
change in resonant frequency and/or amplitude (when using tapping
mode) (STEP 370); [0071] (3) to compensate for this deflection
and/or change in resonant frequency and/or amplitude, the vertical
motion actuator is immediately activated to pull the tip farther up
out of the reservoir, thereby preventing the ink from rushing upon
the tip and down the length of the supporting cantilever (STEP
375); [0072] (4) by adjusting the integral setting of the feedback
loop to accommodate a number of factors including the viscosity of
the ink and the force setpoint, the tip can resonantly engage the
ink reservoir without flooding to create a standing ripple wave
within the ink reservoir (STEP 380); [0073] (5) when the cantilever
deflection and/or resonant frequency and/or amplitude is restored
as the tip is withdrawn from the reservoir, the vertical motion
actuator again drives the tip back down into the reservoir (STEP
385); and [0074] (6) a discrete amount of ink is made to adhere to
the tip when the tip is eventually withdrawn from the ink reservoir
(STEP 390).
[0075] In another embodiment of the invention, a "flooding
compensation" technique can be employed to prevent undesirable
"deposition flooding", which occurs when the tip attempts to
deposit its load of liquid-phase ink onto a substrate which results
in the deposition of a very large dot or the deposition of a line
with a large bulbous structure at one end. Flooding compensation
techniques can be employed for both contact and tapping modes, but
tapping mode usually provides significantly faster response times
and more accurately deposited structures.
[0076] As mentioned above, in a particular embodiment of the
invention, the cantilever operates in a tapping mode (i.e. having a
vibrating tip). This mode is characterized by enhanced feedback
sensitivity, which enables the tip to engage the ink reservoir and
substrate with reduced forces when extracting ink from the
reservoir and depositing the ink onto the substrate, respectively.
Reduced forces preserve the sharpness of the tip and enable the
formation of smaller nanostructures. Also, in the tapping mode, the
operator is able to monitor the mass of ink transferred by the tip
to and from the ink reservoir and the substrate in real time. In
particular, the mass of material acquired by the tip during the
reservoir dipping step, and the mass of material deposited by the
tip onto the substrate can be monitored and calculated in real time
by observing the shifts in the tapping mode resonant frequency
and/or amplitude of the cantilever to which the tip is mounted.
[0077] In one version of this embodiment of the invention, the
nanoassembly system employs an algorithm for re-adjusting the drive
frequency of the vertical motion actuator using the cantilever
feedback loop to more closely match the resonant frequency of the
cantilever (or other vibration element to which the tip is
attached), thereby adjusting for the changes in the cantilever
frequency that results from the tip acquiring and losing mass
during the dipping, transporting, and depositing steps. This
algorithm includes adjusting frequency sweeps to find the newly
acquired resonant frequency of the cantilever; re-calibrating the
scale used to define the desired vibration amplitude of the
cantilever so that the maximum signal derived from the resonant
peak is set to be at full scale; and setting the vertical motion
actuator drive frequency to coincide with the cantilever frequency
to obtain the maximum resonant response or to obtain a particular
desired non-maximum response.
[0078] Various techniques can be employed to enable the tip to
acquire the ink in addition to the method recited in step 310
above. In some embodiments, the tip is exposed to a spray, vapor,
nebulization, plasma, condensing gas, or powder deposition, of the
ink material. Also, another tip or brush-like or sponge-like
instrument may be used to apply or deposit the ink onto the tip.
Furthermore, in yet another embodiment, an ink reservoir is located
adjacent to or inside or on the tip and/or cantilever so as to
enable the ink to move along the outside or through the inside of
the tip and/or cantilever to reach the extremity thereof.
[0079] In still another embodiment, a solvent-laden tip is dipped
into the solid-phase ink in a reservoir via the "solvent assisted"
nanoassembly method. In this embodiment, solid ink in a reservoir
is locally softened or partially dissolved by a small quantity of
solvent transferred by the tip itself to the ink reservoir from a
solvent reservoir disposed proximally thereto. Thusly softened or
dissolved solid ink is then extracted from the ink reservoir by the
tip, as described above.
[0080] In one version of this embodiment, use of a solvent
facilitates extraction of solid-phase ink from a reservoir by the
tip in a tapping mode despite the limited forces that can be
applied by the tapping mode tip to the surface of the reservoir.
During the nanoassembly process, the tip is dipped into a first
reservoir of solvent and then into a second reservoir of solid ink
before the fabrication of each dot or line. Solvent-assisted
techniques according to this embodiment of the invention facilitate
relatively prolonged construction of complex planar designs,
because the duration of fabrication is limited only by the overall
evaporation time of the solvent and not by the evaporation time of
the ink in the ink reservoir. Also, because typical solvents do not
change viscosity during evaporation, many viscosity-related
compensation methods (such as dynamic tuning algorithms) need not
be employed during fabrication. This solvent-assisted technique,
which is applicable both to contact mode and to tapping mode,
enables fabrication of relatively long, substantially uniform lines
that can be drawn continuously until the solvent runs out or dries
out. Alternatively, the solvent can be any liquid-phase material
(such as an etchant) that causes the solid ink in the reservoir to
soften or dissociate or dissolve, so as to become adhered to the
tip.
[0081] Referring to FIG. 4, in a particular embodiment, the
solvent-assisted nanoassembly method includes the following steps:
[0082] (1) initiating vibration of the tip (STEP 410); [0083] (2)
dipping the tip into the liquid-phase solvent reservoir and
extracting a small volume of solvent (STEP 415); [0084] (3) dipping
the solvent-laden tip into the solid-phase ink reservoir, wherein
the solvent immediately begins to soften and/or dissolve the solid
ink in a region near the tip (STEP 420); [0085] (4) extracting a
small quantity of ink from the ink reservoir with the tip (STEP
425); and [0086] (5) depositing the ink onto the substrate with the
tip to form a dot, line, column, or other desirable nanostructure
(STEP 430).
[0087] In one version of this embodiment, one or both of the
solvent reservoir and the ink reservoir are located adjacent or
inside or on the tip and/or cantilever. Also, various techniques
can be employed to enable the tip to deposit the ink in addition to
the method recited in steps 325 and 340 above. For example, in one
embodiment, the extremity of the tip is exposed to an "ink
disassociation enhancing" means that assists in removing the ink
from the tip, thereby causing the ink to preferentially adhere to
the substrate or other pre-existing structure or pattern on the
surface of the substrate. Examples of such means include thermal
sources, such as radiation sources, peltiers, hot plates,
resistively heated zones, and lasers; light sources for fixing
light-curable materials; cooling sources including refrigeration
units and peltiers for fixing "freezeable" ink materials; sources
of a second material that help to remove a first ink material from
the tip and assist in getting that first ink material to
preferentially adhere to the substrate; and high energy sources
located on or in the tip including an electron beam emitter or
laser.
[0088] In one embodiment of the invention, a tip capped with
multiple "sub-tips," located at the extremity of the tip in a
specified arrangement, is used to simultaneously deposit a
complicated pattern in a single deposition step. For example, all
of the required components of a single electron transistor (the
gate, source, drain, and Coulomb blockade island) can be deposited
in one deposition step. In one version of this embodiment, these
multiple "sub-tips" can be located so closely adjacent to one
another as to approximate the topographical features of a
nanostamp. Thus, the tip can be used like a nanostamp, wherein the
relief pattern on the end of the tip can be transferred to the
substrate via an ink pattern. In another version of this
embodiment, multiple sub-tips are used to simultaneously retrieve
and deposit different kinds of ink materials.
[0089] In one embodiment of the invention, grayscale patterns with
varying linewidths and dot sizes can be generated by using a single
dipping operation in the ink reservoir followed by two or more
consecutive deposition steps so that the linewidth and dot size
decrease with greater numbers of deposition steps. Referring to
FIG. 5, four "grayscale" copies of the letter "N" have been
assembled in parallel. Four different line thicknesses were
achieved by using a single ink pool dipping operation followed by
four consecutive deposition steps, one for each "N" being
constructed. As shown in FIG. 5, dot size is uniform and
repeatable, as demonstrated by a series of grayscale lines that
were formed simultaneously, built up from many dots using grayscale
methods. The grayscale method of this embodiment can be employed
with both liquid-phase and solid-phase ink materials.
[0090] As mentioned above, in general, dot size can be reduced by
altering a number of parameters including tip force, tapping or
contact mode, and viscosity or hardness of the ink material. Dot
size can also be substantially reduced by using a "scratch pad"
method wherein the tip is dipped only once into the ink reservoir
and is then used to deposit more than one dot onto the substrate.
Excess ink is removed from the tip via the first several deposited
dots (in a separate "scratch pad" location on or off the
substrate). The final dot to be deposited is much smaller than
could normally be obtained (even when using a very sharp tip).
Thus, a "scratch pad" zone can be used to "pre-deposit" larger
nanostructures, followed by precision deposition of much smaller
nanostructures within the intended fabrication area. This method is
similar to the "grayscale" method described above and shown in FIG.
5, except that the dots deposited in the "scratch pad" zone are
discarded.
[0091] The scratch pad method can be employed with both
liquid-phase and solid-phase ink materials. With a sufficient
number of pre-deposition steps conducted within the scratch pad
zone, a single molecule, a single atom, or a single nanoparticle
may remain at the extremity of the nanotip for subsequent
deposition within the intended fabrication area. The scratch pad
method allows a tip with a relatively large tip radius (several
tens of nanometers in most cases) to deposit a diminishingly small
dot (approximately 2 nm or smaller) of material onto the substrate.
Such a small dot of a metal or other conductive or semiconductive
material is particularly useful as the Coulomb blockade island in a
single electron transistor.
[0092] In one embodiment of the invention, the nanoassembly system
uses the same tip for (1) direct deposition of nanostructures; (2)
imaging of those nanostructures during and after their deposition
(i.e. using the scanning probe microscope in its conventional role
of generating an image of small-scale topography); and (3) imaging
of alignment marks and/or pre-existing structures. The ink material
that becomes adhered to the tip during dipping of the tip into a
reservoir of material only minimally deteriorates the tip's
capability for subsequent high-resolution imaging. In one version
of this embodiment, imaging resolution is improved by dipping the
tip into a reservoir of solvent or light etchant just prior to the
imaging step, thereby removing some or all of the adhered ink
material. In an alternative embodiment, a dedicated tip is used for
imaging during and after the deposition.
[0093] According to embodiments of the invention, in-situ imaging
of the substrate, alignment marks, and pre-existing structures is
conducted periodically during the deposition process to visualize
the deposited structure and adjust the system settings if necessary
to compensate, for example, for relative thermal drift between the
substrate and the tip and accumulation of feedback loop control
errors.
[0094] Many ink materials require curing during or after deposition
to obtain a stable nanostructure. In a preferred embodiment of the
invention, the deposited structures are cured in-situ without
removal of the substrate from the positioning stage using different
curing devices, for example heating devices, such as peltiers that
are also used for cooling of the substrate, as described above,
laser sources, ultra-violet or infrared light sources, electric
current sources, electric voltage sources, or electron-beam
sources.
[0095] To prevent force-related flattening of the deposited
structure after deposition and to facilitate construction of
three-dimensional nanostructures, in one embodiment, the invention
provides for curing or fusing of newly deposited material (while
simultaneously vertically supporting it with the tip) using an
electron beam emitted from the tip of the nanoassembly apparatus.
In this embodiment, a curing process causes a phase transformation
at the very instant that the tip comes into contact with the top
surface of the multi-layer structure. The tip remains in contact
with the deposited material until the end of the phase
transformation, thereby serving as a support for the newly
deposited material to counteract intermolecular and surface forces.
A local emitted electron beam can be used to cure (i.e., crosslink,
fuse or melt) and instantly weld any material that is generally
curable, solidifiable, fusible or meltable using electron beam
radiation. Examples of such inks include nanoparticle inks,
UV-curable polymers, and E-beam-curable polymers. Thus, in this
embodiment of the invention, an electron beam emitted from a
conductive tip instantly welds new materials into place on top of
the substrate and/or on top of pre-existing nanostructures, thereby
facilitating construction of three-dimensional nanostructures. In
this embodiment, high melting point coating materials, such as
silicon carbide, may be used to provide a robust conductive surface
for electron emission from the tip. A suitable range of a
tip-to-substrate current during the electron-beam-assisted
nanoassembly is between approximately 2 to 20 micro-amps. In one
version of this embodiment, in order to maintain a constant flow of
electrons despite substantial fluctuations in the circuit
resistance as the tip makes and breaks contact with the substrate,
a fast-acting current source is used as a source of electrons to be
supplied to the tip. In another version, a voltage source is used.
Preferably, the electron source is triggered on and off by a
threshold applied to the force signal generated by the deflection
of the tip (for contact mode) or the shift in cantilever resonant
amplitude/frequency (for tapping mode) as the tip makes and breaks
contact with the substrate. In another embodiment, an electron
emission source with a relative large (>multimicron) spot size,
located on, inside, adjacent, or some distance away from the tip,
is used to cure the material deposited by the tip.
[0096] In another embodiment, a light source, for example a laser
or non-coherent source, such as a medium-pressure mercury lamp, is
used to cure light-sensitive materials such as UV-curable optical
adhesives deposited by the nanoassembly system. Because some
light-sensitive inks, such as UV-curable optical adhesives, exhibit
exceedingly low vapor pressures and do not readily evaporate or
harden at room temperatures, the viscosity or softness of the inks
can be kept constant for very long periods of time without the
requirement for the reduced fabrication temperatures, thereby
enabling fabrication of complex patterns over long periods of time
in ambient (humid) environments. UV-curable materials generally
include a photoinitiator, i.e. an ingredient that generates free
radicals, which initiates cross-linking between the unsaturation
sites of monomers, oligomers, and polymers. In one version of this
embodiment, a light source located on or near the tip of the
cantilever can be used to intermittently optically cure the
deposited materials in the deposition site each time that the tip
moves away from the deposition site. In another version of this
embodiment, a precisely focused light source, preferably a laser
with small spot size, can be used to cure the light sensitive ink
onto the substrate while the tip is still attached to and
temporarily physically supporting the deposited ink within the
construction zone, thereby enabling the construction of
three-dimensional nanostructures.
[0097] In yet another embodiment, thermally activated tips, for
example resistively heated zones on the cantilever adjacent to the
tip or on apex of the tip, and/or thermally activated substrates,
for example peltier heated/cooled substrates or regions on the
substrate or resistively heated zones on or in the substrate,
provide for in situ intermittent localized thermal curing of
nanostructures at the very instant when the nanostructures are
deposited by the tip onto the substrate or onto other pre-existing
nanostructures, thereby enabling the construction of
three-dimensional nano structures.
[0098] In yet another embodiment, an integrated laser device,
located on or near or some distance away from the tip, enables in
situ intermittent and/or continuous thermal curing, fusing, melting
or solidification of nanostructures while the nanoassembly
proceeds. In one version, an infrared laser-curing device enables
in-situ thermal exposure of the deposited structure at substantial
temperatures without removal of the substrate from the positioning
stage and subsequent re-positioning and alignment during the
nanoassembly process (as would normally be required for curing
using a large hot-plate or oven). In a second version, a laser
device heats the tip (and/or cantilever), which tip then conducts
the heat to the deposited structure. In other words, heating may
occur before, during or after deposition.
[0099] In another version of this embodiment, the laser-curing
device is employed for intermittent thermal curing after deposition
of each layer of the structure to obtain a multi-layer structure.
For example, a laser with a relatively large (multi-micron) spot
size can be used to intermittently thermally cure the deposited
materials in the deposition site each time that the tip moves away
from the deposition site.
[0100] In yet another version, a precisely focused and aimed laser
with a small spot size (sub-micron) can be used to weld and/or
thermally cure the deposited ink materials onto the substrate while
the tip is still attached to and temporarily physically supporting
the deposited ink material within the deposition site, thereby
enabling the construction of three-dimensional nanostructures.
[0101] Referring to FIG. 6, in an illustrative embodiment, a laser
curing system 600 that is employed with the nanoassembly apparatus
100 includes a laser source 610, a laser source rotating mount 620,
a laser beam shutter 630, beam dump 640, focusing lens 650, and a
positioning platform 660. In one embodiment of the invention, a
ytterbium fiber infrared laser, such as the YLD-5000 laser
available from IRE-Poulus Group, emitting at 1060 nm, is used to
supply optical power to a deposition site D on substrate S. In one
version, the maximum optical power supplied is about 5 watts. The
near-collimated laser beam is focused down to illuminate a small
fraction of the deposition site D on substrate S near tip 115 of
cantilever 120 using the focal lens 650 positioned proximately to
substrate S. In one version of the embodiment, the laser curing
system 600 focuses to about a 20.times.10 .mu.m.sup.2 spot within
deposition site D. Timed laser beam exposure is provided by the
laser beam shutter 630, for example, a solenoid-driven shutter,
placed between the laser source 610 and the focusing lens 650. In
one version of this embodiment, 50 ms long laser bursts were
triggered by timing signal output during the deposition sequence.
In another version of this embodiment, timed or pulsed laser beam
exposure is provided by the laser itself (without assistance from a
shutter) to obtain exposure times as short as femtoseconds. In
order to precisely direct the laser beam to deposition site D
without thermally affecting the ink reservoir 190 disposed nearby,
a positioning platform 660 controls the horizontal location,
lateral location, rotation, pitch, and yaw of nanoassembly
apparatus 100 relative to the incoming laser beam. The laser beam
source is mounted on the rotating mount 620 that provides
additional aiming control flexibility. In addition, substrate S is
disposed on top of adjustable positioning stage 111.
[0102] Referring to FIG. 7, for laser-assisted nanoassembly, the
methods of the invention described above include a few extra steps
at the end of each deposition cycle. After the tip extracts inks
from the reservoir and deposits the ink in the deposition site
(STEP 710), instead of returning immediately to the ink reservoir
after deposition of the ink, the tip is directed to move to a
"parking location" where it can not be damaged by the laser beam
(STEP 720). Then, the computer controlled shutter opens (STEP 730)
and the deposition site is exposed to the laser beam, which cures
the most recently deposited nanostructures in the deposition site
(STEP 740). After curing, the shutter is closed (STEP 750); and the
tip returns to the ink reservoir to begin the cycle again (STEP
760).
[0103] It will be apparent to those skilled in the art that various
modifications and variations can be made to the above-described
structure and methodology without departing from the scope or
spirit of the invention.
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