U.S. patent application number 12/465615 was filed with the patent office on 2010-05-06 for scanning probe epitaxy.
This patent application is currently assigned to NORTHWESTERN UNIVERSITY. Invention is credited to Adam B. Braunschweig, Louise R. Giam, Byung Y. Lee, Shifeng Li, Xing Liao, Chang Liu, Chad A. Mirkin, Yuhuang Wang.
Application Number | 20100115672 12/465615 |
Document ID | / |
Family ID | 40852320 |
Filed Date | 2010-05-06 |
United States Patent
Application |
20100115672 |
Kind Code |
A1 |
Mirkin; Chad A. ; et
al. |
May 6, 2010 |
SCANNING PROBE EPITAXY
Abstract
A dual tip probe for scanning probe epitaxy and a method of
forming the dual tip probe are disclosed. The dual tip probe
includes first and second tips disposed on a cantilever arm. The
first and second tips can be a reader tip and a synthesis tip,
respectively. The first tip can remain in contact with a substrate
during writing and provide in situ characterization of the
substrate and or structures written, while the second tip can
perform in non-contact mode to write and synthesis nanostructures.
This feature can allow the dual tip probe to detect errors in a
printed pattern using the first tip and correct the errors using
the second tip.
Inventors: |
Mirkin; Chad A.; (Wilmette,
IL) ; Liu; Chang; (Winnetka, IL) ; Wang;
Yuhuang; (Silver Spring, MD) ; Braunschweig; Adam
B.; (Evanston, IL) ; Liao; Xing; (Evanston,
IL) ; Giam; Louise R.; (Chicago, IL) ; Lee;
Byung Y.; (Seoul, KR) ; Li; Shifeng;
(Carlsbad, CA) |
Correspondence
Address: |
MARSHALL, GERSTEIN & BORUN LLP
233 SOUTH WACKER DRIVE, 6300 SEARS TOWER
CHICAGO
IL
60606-6357
US
|
Assignee: |
NORTHWESTERN UNIVERSITY
Evanston
IL
|
Family ID: |
40852320 |
Appl. No.: |
12/465615 |
Filed: |
May 13, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61052864 |
May 13, 2008 |
|
|
|
61167853 |
Apr 8, 2009 |
|
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Current U.S.
Class: |
850/19 ; 850/33;
850/40 |
Current CPC
Class: |
G01Q 20/04 20130101;
G01Q 70/16 20130101; G01Q 40/00 20130101; G01Q 70/10 20130101; G03F
7/0002 20130101; G01Q 80/00 20130101 |
Class at
Publication: |
850/19 ; 850/33;
850/40 |
International
Class: |
G01Q 60/24 20100101
G01Q060/24 |
Goverment Interests
STATEMENT OF GOVERNMENTAL INTEREST
[0002] This invention was made with government support under Grant
No. N6601-08-1-2044 awarded by the Space and Navel Warfare Systems
Center. The government has certain rights in the invention.
Claims
1. A dual tip micro probe, comprising: a micro cantilever arm
comprising first and second arm ends and a major axis disposed
along a length of the cantilever arm between the first and second
arm ends; a first tip disposed on the arm adjacent to the second
arm end; and a second tip disposed on the arm adjacent to and in
line with the first tip along the major axis.
2. The dual tip probe of claim 1, wherein the arm comprises first
and second arm sections disposed between the first and second arm
ends, the first arm section having a width greater than the second
arm section, and the first and second tips being disposed on the
second arm section.
3. The dual tip probe of claim 1, wherein the first and second tips
are inverted pyramids comprising a base and an apex, wherein the
base is operatively coupled to the arm.
4. The dual tip probe of claim 3, wherein the second tip further
comprises an aperture formed at the apex.
5. The dual tip probe of claim 4, wherein the aperture has a
diameter in a range of about 5 nm to about 200 nm.
6. The dual tip probe of claim 1, wherein the second tip is a
hollow tip.
7. The dual tip probe of claim 1, wherein the first tip and the
second tip have the same thickness.
8. The dual tip probe of claim 1, wherein the first tip has a first
thickness and the second tip has a second thickness different from
the first thickness.
9. The dual tip probe of claim 1, wherein the first tip is a
non-synthesis tip and the second tip is a synthesis tip for forming
nanostructures on or in a substrate.
10. The dual tip probe of claim 9, wherein the first tip is a
surface topology measurement tip.
11. The dual tip probe of claim 1, wherein the second tip is a
synthesis tip selected from the group consisting of an aperture
probe synthesis tip, a catalyst tipped synthesis tip, a resistively
heated high temperature synthesis tip, an elastomer gel synthesis
tip, a polymer synthesis tip, and an electric field control
synthesis tip.
12. The dual tip probe of claim 11, wherein the synthesis tip is a
high temperature synthesis tip and the high temperature synthesis
tip comprises a resistive heater disposed on the tip.
13. The dual tip probe of claim 1, wherein the first tip is formed
from a material selected from the group consisting of silicon,
silicon oxide, silicon nitride, gold, silver, copper, and
tungsten.
14. The dual tip probe of claim 1, wherein the second tip is formed
of a material selected from the group consisting of doped
polysilicon, a metal, a hard metal, and a metal oxide.
15. The dual tip probe of claim 14, wherein the metal is selected
from the group consisting of Au, Ag, Al, Ni, Fe, Pt, Os, Ru, Ir,
In, W, and Cr.
16. The dual tip probe of claim 14, wherein the hard metal is
selected from the group consisting of TiN, TiC, WC, and TaN.
17. The dual tip probe of claim 14, wherein the metal oxide is
selected from the group consisting of IrO.sub.2 and InO.
18. The dual tip probe of claim 1, wherein the arm is formed from a
material selected from the group consisting of silicon, silicon
nitride, silicon oxide, silver, gold, aluminum, tungsten, and
copper.
19. The dual tip probe of claim 1, wherein the first and second
tips are separated by a distance in a range of 1 .mu.m to 50 .mu.m,
measured as the distance from the apex of the first tip to the apex
of the second tip.
20. A dual tip micro probe, comprising: a micro cantilever arm
comprising first and second arm ends; a first tip disposed on the
arm, adjacent to the second arm end, wherein the first tip is a
non-synthesis tip; and a second tip disposed on the arm adjacent to
the first tip, wherein the second tip is a synthesis tip.
21. The dual tip micro probe of claim 20, wherein the cantilever
arm is adapted to bend such that the first tip will contact a
substrate while the second tip will remain disposed above the
substrate.
22. The dual tip probe of claim 20, wherein the arm comprising
first and second arm sections disposed between the first and second
arm ends, the first arm section having a width greater than the
width of the second arm section, and the first and second tips
being disposed on the second arm section.
23. The dual tip probe of claim 20, wherein the first and second
tips are inverted pyramids comprising a base and an apex, wherein
the base is operatively coupled to the arm.
24. The dual tip probe of claim 23, wherein the second tip further
comprises an aperture formed at the apex.
25. The dual tip probe of claim 24, wherein the aperture has a
diameter in a range of about 5 nm to about 200 nm.
26. The dual tip probe of claim 20, wherein the second tip is a
hollow tip.
27. The dual tip probe of claim 20, wherein the first tip and the
second tip have the same thickness.
28. The dual tip probe of claim 20, wherein the first tip has a
first thickness and the second tip has a second thickness different
from the first thickness.
29. The dual tip probe of claim 20, wherein the second tip is a
synthesis tip selected from the group consisting of an aperture
probe synthesis tip, a catalyst tipped synthesis tip, a resistively
heated high temperature synthesis tip, an elastomer gel synthesis
tip, a polymer synthesis tip, and an electric field control
synthesis tip.
30. The dual tip probe of claim 29, wherein the synthesis tip is a
high temperature synthesis tip and the high temperature synthesis
tip comprises a resistive heater disposed on the tip.
31. The dual tip probe of claim 20, wherein the first tip is formed
from a material selected from the group consisting of silicon,
silicon oxide, silicon nitride, gold, silver, copper and
tungsten.
32. The dual tip probe of claim 20, wherein the second tip is
formed of a material selected from the group consisting of doped
polysilicon, doped diamond, a metal, a hard metal, and a metal
oxide.
33. The dual tip probe of claim 32, wherein the metal is selected
from the group consisting of Au, Al, Ag, Ni, Fe, Pt, Os, Ru, In,
Ir, W and Cr.
34. The dual tip probe of claim 32, wherein the hard metal is
selected from the group consisting of TiN, TiC, WC, and TaN.
35. The dual tip probe of claim 32, wherein the metal oxide is
selected from the group consisting of InO and IrO.sub.2.
36. The dual tip probe of claim 20, wherein the arm is formed from
a material selected from the group consisting of silicon, silicon
nitride, silicon oxide, silver, gold, aluminum, tungsten, and
copper.
37. The dual tip probe of claim 20, wherein the first and second
tips are separated by a distance in a range of 1 .mu.m to 50 .mu.m,
measured as the distance from the apex of the first tip to the apex
of the second tip.
38. A method of making a dual tip probe, comprising: forming a
first opening in a substrate; forming a second opening adjacent to
the first opening in the substrate; forming a first tip layer over
the substrate including at least the first opening; removing at
least a portion of the first tip layer disposed outside the first
opening, to form the first tip; forming a second tip layer over the
substrate including at least the second opening; removing at least
a portion of the second tip layer disposed outside the second
opening, to form the second tip; forming a cantilever arm layer
over the substrate and the first and second tips, to form the dual
tip probe comprising a cantilever arm connected to the first and
second tips; and separating the dual tip probe from the
substrate.
39. The method of claim 38, comprising forming the first and second
openings as pyramidal openings.
40. The method of claim 38, comprising etching the substrate to
form the first and second openings.
41. The method of claim 38, comprising forming the first tip layer
from a material selected from the group consisting of silicon,
silicon oxide, silicon nitride, gold, silver, copper, and
tungsten.
42. The method of claim 38, comprising forming the second tip layer
from a material selected from the group consisting of doped
polysilicon, doped diamond, a metal, hard metal, or metal
oxide.
43. The method of claim 38, wherein the metal is selected from the
group consisting of Au, Al, Ag, Ni, Fe, Pt, Os, Ru, In, W, Ir, and
Cr.
44. The method of claim 38, comprising forming the cantilever arm
layer from a material selected from the group consisting of
silicon, silicon nitride, silicon oxide, silver, gold, aluminum,
tungsten, and copper.
45. The method of claim 38, further comprising removing at least a
portion of the cantilever arm layer disposed in the first and
second openings.
46. The method of claim 38, further comprising forming a
sacrificial layer over the substrate including the first and second
openings before forming the first and second tip layers, and
further comprising etching the sacrificial layer to separate the
dual tip probe from the substrate.
47. The method of claim 38, comprising forming the arm layer by
sequentially depositing an electrical biasing layer, an insulating
layer, and conductive layer onto the substrate including the first
and second tips.
48. The method of claim 38, further comprising wire bonding a
resistive heater on the second tip by wire bonding.
49. A method of in situ correction of a printed indicia using a
dual tip probe, the method comprising: characterizing a substrate
having printed indicia comprising an error, the error comprising a
discrepancy between the printed indicia and a predetermined pattern
for printed indicia, the discrepancy comprising a printing omission
or an additional feature, using a first tip of a dual tip probe
comprising first and second tips disposed on a cantilever arm to
detect the error in the printed indicia; and correcting the error
in the printed indicia using the second tip by printing a
correction indicia spatially corresponding to the printing omission
or removing the additional feature.
50. The method of claim 49, further comprising using the second tip
to print the printed indicia before characterizing the substrate
using the first tip.
51. The method of a claim 49, wherein one or both of the printed
indicia and the predetermined pattern comprises a circuit.
52. The method of claim 51, comprising characterizing the circuit
using the first tip, wherein the error is a gap in the circuit; and
correcting the error by forming a conductive structure in the gap
using the second tip to close the gap.
53. A method of calibrating a relationship between applied force
and distance between a synthesis tip and a substrate, the method
comprising: determining a difference between the thicknesses of a
reader tip and a synthesis tip of a dual tip probe comprising the
reader tip and the synthesis tip disposed on a cantilever arm;
contacting the substrate with the reader tip, wherein the synthesis
tip does not contact the substrate; applying a force to the
cantilever arm to bend the cantilever arm and displace the
synthesis tip toward the substrate; determining the amount of
bending of the cantilever arm; and calculating the distance between
synthesis tip and the substrate using the thickness difference and
the amount of cantilever arm bending.
54. A method of adjusting the distance between a synthesis tip of a
dual tip probe and a substrate, the method comprising: contacting a
substrate with a non-synthesis tip of a dual tip probe comprising
the non-synthesis tip and a synthesis tip disposed on a cantilever
arm, the non-synthesis tip and the synthesis tip disposed in line
with each other on a major axis of the cantilever arm; and applying
a force to the cantilever arm to bend the cantilever arm and
displace the synthesis tip toward the substrate.
55. The method of claim 54, further comprising adjusting the amount
of force applied to the cantilever arm to adjust the distance
between the synthesis tip and the substrate based on a
predetermined relationship between the amount of force applied to
the cantilever arm after contacting the substrate with the
non-synthesis tip and the distance between the synthesis tip and
the substrate.
56. The method of claim 55, wherein determining the predetermined
relationship between the amount of force applied to the cantilever
arm after contacting the substrate with the non-synthesis tip and
the distance between the synthesis tip and the substrate is
comprises calculating the distance between the synthesis tip and
the substrate using the amount of cantilever arm bending and a
difference in thickness of the synthesis and non-synthesis tip.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The benefit under 35 U.S.C. .sctn.119 (e) of U.S.
Provisional Patent Application No. 61/052,864 filed May 13, 2008,
and U.S. Provisional Patent Application No. 61/167,853, filed Apr.
8, 2009, the disclosures of which are incorporated herein by
reference, is hereby claimed.
BACKGROUND
[0003] 1. Field of the Disclosure
[0004] The disclosure generally relates to a probe for scanning
probe epitaxy, methods of making the probe, and methods of using
the probe to form nanostructures. In particular, the disclosure
relates to a probe for scanning probe epitaxy having a dual tip
architecture. The disclosure further relates to a method of forming
nanostructures using block copolymers that phase separate to form
nanostructure templates.
[0005] 2. Brief Description of Related Technology
[0006] There a variety of known tip-based methods of synthesizing
nanostructures on a surface. Different capabilities are needed for
the synthesis of different nanostructures. For example, for the
synthesis of quantum dots, the ability to directly delivery a
reactive chemical reagent to a second reagent on a surface in order
to make a binary structure is generally needed. The synthesis of
quantum dots and nanoparticles can also utilize the application of
an electric field that transforms a tip into a nanoevaporator
capable of depositing nanoscopic amounts of a high vapor pressure
material on a surface. Control of the tip temperature up to
hundreds of degrees can be utilized to facilitate the direct
catalytic growth of solid state nanostructures like carbon
nanotubes and semiconductor nanowires. There are no currently
available methods for realizing such capabilities with commercially
or even academic laboratory-available scanning probe systems.
SUMMARY
[0007] In accordance with an embodiment of the invention, a dual
tip micro probe includes a micro cantilever arm comprising first
and second arm ends and a major axis disposed along a length of the
cantilever arm between the first and second arm ends, a first tip
disposed on the arm adjacent to the second arm end, and a second
tip disposed on the arm adjacent to and in line with the first tip
along the major axis.
[0008] In accordance with another embodiment of the invention a
dual tip micro probe includes a micro cantilever arm comprising
first and second arm ends, a first tip disposed on the arm,
adjacent to the second arm end, wherein the first tip is a
non-synthesis tip, and a second tip disposed on the arm adjacent to
the first tip, wherein the second tip is a synthesis tip.
[0009] In accordance with yet another embodiment of the invention,
a method of making a dual tip probe includes forming a first
opening in a substrate, forming a second opening adjacent to the
first opening in the substrate, and forming a first tip layer over
the substrate including at least the first opening. The method
further includes removing at least a portion of the first tip layer
disposed outside the first opening to form the first tip, forming a
second tip layer over the substrate including at least the second
opening, removing at least a portion of the second tip layer
disposed outside the second opening to form the second tip, forming
a cantilever arm layer over the substrate and the first and second
tips to form the dual tip probe comprising a cantilever arm
connected to the first and second tips, and separating the dual tip
probe from the substrate.
[0010] In accordance with an embodiment of the invention a method
of in situ correction of a printed indicia using a dual tip probe
includes characterizing a substrate having printed indicia
comprising an error, the error comprising a discrepancy between the
printed indicia and a predetermined pattern for printed indicia,
the discrepancy comprising a printing omission, or an additional
feature (e.g., an extra printed feature not corresponding to the
predetermined pattern or extraneous feature not corresponding to
the predetermined pattern), using a first tip of a dual tip probe
comprising first and second tips disposed on a cantilever arm to
detect the error in the printed indicia, and correcting the error
in the printed indicia using the second tip by printing a
correction indicia spatially corresponding to the printing omission
or removing the additional feature.
[0011] In accordance with an embodiment of the invention, a method
of calibrating the force-distance relationship between a synthesis
tip and a substrate includes determining a difference between the
thicknesses of a reader tip and a synthesis tip of a dual tip probe
comprising the reader tip and the synthesis tip disposed on a
cantilever arm, contacting the substrate with the reader tip,
wherein the synthesis tip does not contact the substrate, applying
a force to the cantilever arm to bend the cantilever arm and
displace the synthesis tip toward the substrate, determining the
amount of bending of the cantilever arm, and calculating the
distance between synthesis tip and the substrate using the
thickness difference and the amount of cantilever arm bending.
[0012] In accordance with yet another embodiment of the invention,
a method of adjusting the distance between a synthesis tip of a
dual tip probe and a substrate includes contacting a substrate with
a non-synthesis tip of a dual tip probe comprising the
non-synthesis tip and a synthesis tip disposed on a cantilever arm,
the non-synthesis tip and the synthesis tip disposed in line with
each other on a major axis of the cantilever arm, and applying a
force to the cantilever arm to bend the cantilever arm and displace
the synthesis tip toward the substrate.
[0013] In accordance with an embodiment of the invention a method
of forming a nanostructure, the method includes: patterning a block
copolymer on a substrate, wherein the block copolymer phase
separates to form a nanostructure template, loading the
nanostructure template with a nanostructure precursor material, and
removing the polymer to form the nanostructure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIGS. 1A and 1B are schematics of dual tip probes in
accordance with embodiments of the disclosed invention. FIG. 1B
illustrates a dual tip probe having strain gauges. FIG. 1C is a
schematic of a dual tip probe in accordance with an embodiment of
the disclosed invention illustrating the relationship between probe
dimensions, cantilever bending, and distance between the second tip
and the substrate.
[0015] FIGS. 2A and 2B are scanning electron microscopy (SEM)
images of dual tip probes in accordance with embodiments of the
disclosed invention.
[0016] FIGS. 3A-3B are AFM images of in situ correction of a
pattern using a dual tip probe in accordance with an embodiment of
the disclosed invention. FIG. 3D is AFM height images of the
patterns of FIG. 3A. FIG. 3E is an SEM image of the pattern of FIG.
3A and an energy dispersive x-ray spectrum of the pattern.
[0017] FIG. 4A is an SEM image of an ultra sharp tip having a 10 nm
radius of curvature. FIG. 4B is a graph showing the convoluted tip
results of the tip of FIG. 4A. FIG. 4C is SEM images of a sharp tip
before and after writing, showing the wear on a tip that occurs
during writing.
[0018] FIG. 5 is a schematic of a cantilever arm of a dual tip
probe in accordance with an embodiment of the disclosed invention
having two sections of different widths and lengths.
[0019] FIG. 6A is an optical microscopy image of a dual tip probe
in accordance with an embodiment of the disclosed invention having
a cantilever arm with two sections of different widths. FIG. 6B is
an optical microscopy image of a dual tip probe in accordance with
an embodiment of the disclosed invention having a rib disposed
between the first and second tips.
[0020] FIG. 7 is an SEM image of a dual tip probe in accordance
with an embodiment of the invention having an aperture in the
second tip.
[0021] FIG. 8A is an SEM image of a thermal dual tip probe in
accordance with an embodiment of the claimed invention. FIG. 8B is
an optical image of the dual tip probe of FIG. 8A. FIG. 8C is a
simulated model of a thermal dual tip probe in accordance with an
embodiment of the invention. FIG. 8D is a simulated temperature
distribution of a thermal dual tip probe. FIG. 8E is a thermal
microscopy image of a thermal tip of a thermal dual tip probe. FIG.
8F is a graph of tip temperature verses voltage comparing
simulation results with measured results. FIG. 8G is a temperature
distribution obtained from the simulation shown in FIG. 8F. FIG. 8H
is a graph showing the maximum temperature verses the voltage bias
for a thermal dual tip probe. FIG. 81 is a schematic of a thermal
tip having a resistive heater disposed on the tip.
[0022] FIG. 9A is a cross-sectional schematic of an electric field
controlled dual tip probe in accordance with an embodiment of the
disclosed invention, showing a wire disposed within the hollow tip
down to a point in proximity to the aperture at the apex of the
tip, and an evaporatable material disposed within the hollow
portion of the tip. FIG. 9B is a schematic of evaporation of a
metal from electric field controlled dual tip probe in accordance
with an embodiment of the disclosed invention as a result of an
applied electric field between the tip and surface.
[0023] FIG. 10A is a schematic of a bending of a dual tip
cantilever in connection with force-distance calibration of a dual
tip probe in accordance with an embodiment of the disclosed
invention. FIG. 10B is graph showing the resulting force distance
curve.
[0024] FIG. 11A-11C are graphs showing ANSYS modeling of the
electric field and thermal gradients as a function of distance
between the tip and the substrate. FIG. 11D is a graph showing
ANSYS modeling of temperature diffusion from a tip.
[0025] FIGS. 12A and 12B are graphs illustrated the dependence of
the thermal gradient on distance between the tip and substrate.
FIG. 12A illustrates the change of electric field intensity with
distance from the substrate under fixed voltage bias (U=20V). FIG.
12B illustrates the voltage bias required to achieve an electric
field intensity of 5.times.10.sup.9 V/m for a given distance
between the tip and the substrate.
[0026] FIGS. 13A-13D are graphs showing the electric field gradient
as a function of tip surface topology.
[0027] FIGS. 14A-14D are force-distance curves illustrating how
stiffness can affect the performance of a dual tip probe in
accordance with an embodiment of the invention.
[0028] FIG. 15A is a schematic illustrating the synthesis of a
carbon nanotube (CNT) from a heated tip. FIG. 15B is a schematic
illustrating synthesis of a CNT from a tip using a heated
substrate.
[0029] FIG. 16A is a schematic illustrating formation of
nanostructures using probe assisted deliver of chemical reagents to
nanoreactors on a surface. FIG. 16B is an AFM height image of
nanowells formed by phase separation of immiscible polymers. FIG.
16C is an AFM height image of nanowells formed using electron-beam
lithography. FIG. 16D is an SEM image showing nanowell formation by
oxidation of anodic aluminum oxide.
[0030] FIG. 17 is a schematic generally illustrating the mold and
transfer method for forming dual tip probes in accordance with an
embodiment of the disclosed invention.
[0031] FIG. 18 is a schematic illustrating a method of forming a
dual tip probe in accordance with an embodiment of the disclosed
invention.
[0032] FIG. 19A is a schematic illustrating formation of a
nanoparticle array using block copolymer nanostructure templates in
accordance with an embodiment of the disclosed invention. FIG. 19B
is an AFM image of the block copolymer nanostructure template
formed in accordance with the method illustrated in FIG. 19A. FIG.
19C is SEM images of a nanoparticle array formed in accordance with
the method illustrated in FIG. 19A.
[0033] FIG. 20A is a schematic illustrating formation of a nanowire
using block copolymer nanostructure templates in accordance with an
embodiment of the disclosed invention. FIG. 20B is an AFM image of
the block copolymer nanostructure template formed in accordance
with the method illustrated in FIG. 20A. FIG. 20C is an SEM image
of a nanowire formed in accordance with the method illustrated in
FIG. 20A.
[0034] FIG. 21 is a schematic of the mold and transfer method for
forming a dual tip probe in accordance with the method described in
Example 1.
[0035] FIG. 22 is an SEM of a dual tip probe illustrating a dual
tip probe formed in accordance with the method described in Example
1.
[0036] FIG. 23 is an SEM image of an array of silicon nitride dual
tip probes formed in accordance with the method described in
Example 2.
[0037] FIGS. 24A and 24B are atomic force microscopy images
comparing the imaging capability of a dual tip probe in accordance
with the disclosed invention (FIG. 24B) to a conventional single
tip (FIG. 24A).
[0038] FIG. 25A is an AFM height image of a 10 particle per line
gold nanoparticle pattern formed using an AFM tip. FIG. 25B is an
AFM image of two gold hexagon shaped structures formed by pulsed
evaporation of gold from an AFM tip.
[0039] FIGS. 26A-26D are SEM images of Au patterned structures
generated by applying a 20 V tip bias voltage to a gold coated AFM
tip. FIG. 26A is an SEM image of Au nanoparticles evaporated onto a
silicon dioxide surface. FIG. 26B is a low SE mode image showing
the raised topology of a 3 line pattern of Au. FIGS. 26C and 26D
show Au patterns formed by varying the pulse rates of the applied
voltage. FIG. 26C has 10 s.sup.-1 pulse rate and FIG. 26D has a 4
s.sup.-1 pulse rate.
[0040] FIG. 27 is a schematic of an embodiment of hardware which
can be used for scanning pulsed evaporation of metal from a probe
tip.
DETAILED DESCRIPTION
[0041] Scanning Probe Epitaxy (SPE) is the atom by atom growth of
nanostructures from a surface through the controlled delivery of
chemical reagents to that surface under environmental control. SPE
can enable the tip-based synthesis of carbon nanotubes,
semiconductor nanowires, nanoparticles, quantum dots, and other
printed indicia or patterns with control over the architecture
(e.g., length, diameter, and composition) of each nanostructure or
pattern and control over the orientation and spacing of the
nanostructures on a surface. Tip-based synthesis reactions can
occur, for example, on a substrate where the tip delivers the
chemical reagents to the substrate. Alternatively, the reaction can
occur at the tip surface where reagents in the gas phase are
delivered to the tip and a controlled pulse of energy can release
the nanostructure from the tip to a surface site or substrate of
interest.
Dual Tip Probe Structure
[0042] Referring to FIGS. 1A and 1B and 2, a micro probe 10 having
a dual tip architecture can include a cantilever arm 12, a first
tip 14 disposed on the cantilever arm 12, and a second tip 18
disposed on the cantilever arm 12 adjacent to the first tip 14. The
apices of the first and second tips can be formed on independent
monolithic base structures connected to the cantilever arm, or the
apices of the first and second tips can be formed on a common
monolithic base structure (see e.g., FIG. 7). The micro probe, also
referred to herein as "the dual tip probe" can include, for
example, a non-synthesis tip, such as a reader tip, and a synthesis
tip (e.g., one that creates features by addition, subtraction, or
alteration of material) as the first and second tips, as shown in
FIG. 1A. Alternatively, the dual tip can include two reader tips or
two synthesis tips. Any other known or suitable tip types can be
included as one or both of the tips on the dual tip probe. The dual
tip probe can further include, for example, one or more stiffness
ribs (as shown in FIG. 6B) disposed on the cantilever arm, for
example, between the first and second tips The dual tip probe can
further include one or more strain gauges (as shown in FIG. 1B)
disposed on the cantilever arm.
[0043] The inclusion of both a reader tip and a synthesis tip on a
single cantilever arm can allow for the simultaneous or
substantially simultaneous (a) measurement of the topology of a
surface and (b) synthesis of nanostructures or printing of indicia
on the substrate. This can allow for in situ correction of an
error. For example, the dual tip probe can be used to detect, with
the first or reader tip, an error in a printed indicia. The error
can be, for example, an omission in the printed indicia. The error
can also be, for example, an additional printed feature, such as an
extra printed feature or an extraneous feature not introduced via
printing (e.g., a flaw in the substrate, or other extraneously
introduced material). The second tip can then be used to correct
the error either by printing a correction indicia spatially
corresponding to the printing omission or by removing the
additional feature. An extra printed feature can be removed, for
example, by etching the extra printed feature, for example, by
depositing an etchant with the second tip onto the extra printed
feature. Where the printed indicia is a circuit, for example, the
error can be a gap in the circuit. For example, where the printed
indicia is a circuit and the error is a gap in the circuit, the
second tip can be used to print or form a conducting nanowire in
the gap to reconnect the circuit. The error in a circuit can also
be, for example, an extra conductive wire or dot that erroneously
couples portions of the circuit. This error can be corrected, for
example, by depositing with the second tip an etchant or other
material to remove the extra wire or dot from the circuit pattern.
Any known tip based printing and removal methods can be used for
correction of a detected error in a printed indicia. For example,
it is well known that certain metal salts (e.g., metal halides) are
more volatile than the parent metal. Thus, extra printed metal or
extraneous metal can be removed by depositing a suitable material
that reacts with the metal to form a volatile species that
evaporates from the substrate.
[0044] The dual tip probe can also be used, for example, for in
situ correction of a printed indicia while printing the indicia
using the second tip. A printed indicia can be printed using the
second tip and simultaneously or substantially simultaneously
characterized using the first tip to detect errors in the printed
indicia. The writing of the printed indicia and the detection of
errors using the synthesis and reader tips, respectively, can also
occur, for example, sequentially. Then, as described above, the
error can be corrected by reprinting with the second tip.
[0045] FIG. 3 illustrates in situ correction of a pattern using the
dual tip probe. As illustrated in FIGS. 3A and 3B, a first Au
pattern was printed using the second tip. Characterization of the
Au pattern using the first tip detected error in the patterns,
namely omission of printed Au between the square patterns. As
illustrated in FIG. 3C, this error was corrected using the second
tip by printing the additional Au patterns.
[0046] The dual tip structure can also allow for simultaneously
working in both contact and non-contact mode. For example, the
synthesis tip can operate in a non-contact mode, while the reader
tip can be in contact with the surface and operate in contact mode.
This can aid in preventing synthesis tip wear due to contact with
the substrate. This can be especially useful with sharp synthesis
tips that are more susceptible to wear when operated in contact
mode. FIG. 4 shows a sharp tip both before and after writing to
illustrate the wear on the tip. Tip wear can diminish the
resolution of the tip.
[0047] Referring back to FIGS. 1 and 2, the cantilever arm can
extend the entire length of the micro probe and operatively couple
the first and second tips. The cantilever arm can be formed, for
example, from silicon nitride, silicon oxide, or polysilicon. The
cantilever arm can also be formed, for example, from a metal, such
as, silver, gold, aluminum, tungsten, and copper. The cantilever
arm can be designed to bend in response to an applied force. The
cantilever arm can further include electric leads for applying a
bias and/or electrical pulses to an electric field controlled or
thermal synthesis tip. The electric leads can be, for example,
printed onto the cantilever arm.
[0048] The cantilever arm has first and second ends, with the first
and second tips disposed adjacent the second end. The cantilever
arm can have any suitable length, for example, in a range of 100 to
500 .mu.m. Other suitable lengths include, for example, ranges of
100 .mu.m to 400 .mu.m, 150 .mu.m to 350 .mu.m, 100 .mu.m to 300
.mu.m, 200 .mu.m to 500 .mu.m, 200 .mu.m to 400 .mu.m, and 200
.mu.m to 300 .mu.m. The length can be for example, about 100, 150,
200, 250, 300, 350, 400, 450, or 500 .mu.m. The cantilever arm can
have any suitable thickness, for example, in a range of 1 .mu.m to
100 .mu.m. Other suitable thickness include ranges of 10 .mu.m to
80 .mu.m, 20 .mu.m to 60 .mu.m, and 30 .mu.m to 50 .mu.m. The
thickness can be for example, about 1, 5, 10, 15, 20, 25, 30, 35,
40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100 .mu.m.
[0049] Referring to FIG. 2, the cantilever arm can have a constant
width along the entire length of the cantilever arm. Alternatively,
the cantilever arm can have a varied width comprising two, three,
four or more discrete sections, or tapered sections or cantilever
arm. For example, as shown in FIGS. 5 and 6A, the cantilever arm 26
can have two sections 20 and 22 disposed between the first and
second ends 24 and 28, respectively. The first and second tips 30
and 32, respectively, can be disposed, for example, on the second
section 22. The first section 20 of the cantilever arm 26 can have
a width W1 and a length L1, and the second section 22 of the
cantilever arm can 26 have a width W2 and a length L2. The length
L1, L2 and width W1, W2 of the first and second sections 20 and 22
can be different. For example, the length L1 and width W1 of the
first section 20 can be greater than the length L2 and width W2 of
the second section 22. The length L1 of the first section 20 can
be, for example, in a range of 50 .mu.m to 400 .mu.m. Other
suitable lengths L1 include, for example, 50 .mu.m to 350 .mu.m, 75
.mu.m to 200 .mu.m, and 100 .mu.m to 300 .mu.m. Length L1 can be,
for example, about 50, 60, 70, 80, 90, 100, 110, 120, 130, 140,
150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270,
280, 290, 300, 310, 320, 330, 340, 350, 360, 370, 380, 390, or 400
.mu.m. The length L2 of the second section 22 can be, for example,
in a range of 25 .mu.m to 100 .mu.m. Other suitable lengths L2
include, for example, 30 .mu.m to 70 .mu.m, 40 .mu.m to 60 .mu.m,
and 25 .mu.m to 50 .mu.m. Length L2, can be, for example, about 25,
30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100
.mu.m. The widths W1, W2 of the first and second sections 20 and 22
can be, for example, in a range of 5 .mu.m to 100 .mu.m. Other
suitable widths include, for example, 10 .mu.m to 90 .mu.m, 20
.mu.m to 80 .mu.m, 30 .mu.m to 70 .mu.m, and 40 .mu.m to 60 .mu.m.
One or both of the widths, W1, W2, can be, for example, about 5,
10, 15, 20, 25, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95,
or 100 .mu.m. A micro probe having a cantilever arm 26 having a
second section with a narrower width W2 than the width W1 of a
first section can have increased mechanical stiffness between first
and second tips as compared to a probe having a uniform width along
the entire cantilever. Variation of the width of sections of the
cantilever arm can be used as means for varying the stiffness of
the probe in selected regions, and finer control over the bending
of the cantilever arm after contact of the first tip with the
surface end during increased applied force to bring the second tip
in closer proximity to the surface. Variation of the other
dimensions of the cantilever arm include, for example, total
cantilever arm length, first and second section length L1 and L2,
cantilever arm thickness and cantilever arm materials can also
affect stiffness and bending performance. Thus, these variables can
be modified to achieve the desired force-distance relationship for
a dual tip probe.
[0050] With reference to FIG. 5, for example, the first tip 30 is
disposed adjacent to the second end 28 of the cantilever arm 26.
The first tip 30 can be designed, for example, as a non-synthesis
or reader tip. For example, the first tip can be a surface topology
measurement tip. The reader tip can operate, for example, in
contact mode to measure and characterize the topology of a surface
of a substrate and/or structures printed on the substrate. The
first tip 30 can be brought into contact with a substrate, for
example, by applying a force on the cantilever arm 26 to bend the
cantilever arm and displace the first tip 30 towards the surface.
The first tip can be formed, for example, from a dielectric
materials such as, for example, silicon, silicon nitride, or
silicon dioxide. The first tip can also be formed, for example,
from a metal, such as Au, Ag, Cu, and W. The first tip can have a
thickness in a range of 1 .mu.m to 20 .mu.m, for example. Other
suitable thickness included for example, about 1, 2, 3, 4, 5, 6, 7,
8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 .mu.m. The
first tip can have, for example, a pyramidal shape with a base
disposed on the cantilever arm and an apex terminating in a point,
and thus the thickness is measured as the distance from the base of
the tip disposed on the cantilever arm to the apex. The point can
be, for example, a sharp point or a rounded, e.g. spherical point.
The tip can also have a cylindrical shape, wherein the major axis
of the cylinder projects out from the cantilever arm, e.g. in
perpendicular fashion. The first tip can be any known or suitable
reader type tip having any other suitable geometry. For example,
the first tip can be an atomic force microscopy tip or a scanning
microscopy tip. The first tip can be for example a solid tip or a
hollow tip, and preferably is a solid tip.
[0051] The second tip 32 is disposed on the cantilever arm 26
adjacent to the first tip 30. The second tip 32 can be adjacent the
first tip 30 and in line with the major axis of the cantilever arm
26. In the alternative, the second tip 32 can be offset from the
first tip 30 toward the first end 24 of the cantilever arm 26 by a
distance x, as illustrated in FIG. 5. The distance x can be in a
range for example, of 1 .mu.m to 50 .mu.m. Other suitable distances
x include, for example, 1 .mu.m to 40 .mu.m, 5 .mu.m to 30 .mu.m,
and 10 .mu.m to 20 .mu.m. The distance x can be, for example, about
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,
37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or 50
.mu.m.
[0052] The second tip can be designed as a synthesis tip for
additive fabrication, such as synthesizing nanostructures and/or
printing (e.g. depositing) indicia or patterns. The second tip can
also be designed as a tip for subtractive fabrication to remove
features from a substrate. For example, the second tip can be used
to deposit an etchant to remove a portion or a feature of a
substrate. The tip can also be used, for example, to deposit a
material that reacts with a metal feature on a substrate that
reacts with the metal to form a volatile species, such as a metal
salt, for example, a metal halide, that evaporates from the
substrate. As used herein, "synthesis tip" refers to a tip with
either additive fabrication capabilities (forming structures onto a
substrate), subtractive fabrication capabilities (i.e. removing
structures from a substrate), or alteration capabilities (e.g.,
reaction, phase change, magnetic properties).
[0053] The second tip can operate in either contact mode, in which
the second tip is in contact with a substrate, or preferably in
non-contact mode, in which the second tip is disposed above the
substrate. The second tip can be formed, for example, from a
conductive material such as, for example, doped polysilicon, doped
diamond, a metal, a hard metal, or a metal oxide. Suitable metals
include Au, Al, Ni, Fe, Pt, Os, Ru, Ir, In, W, Ag, and Cr. Suitable
hard metals include TiN, TiC, WC, and TaN. Suitable metal oxides
can be, for example, InO and IrO.sub.2. The second tip can have a
thickness in a range of 1 .mu.m to 20 .mu.m, for example. Other
suitable thickness include, for example, about 1, 2, 3, 4, 5, 6, 7,
8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 .mu.m. The
first and second tips can have the same thickness or can have
different thicknesses. For example, the first tip can be thicker
than the second tip (see, for example, FIG. 1a). Referring to FIGS.
2 and 4, the tips, and preferably the second tip, can have a
pyramidal shape with a base disposed on the cantilever arm and an
apex terminating at a point. The point can be, for example, a sharp
point or a rounded spherical point. The second tip can also have,
for example, a cylindrical or spherical shape. The second tip can
have a rough or a smooth surface. For example, a second tip can
have a pyramidal shape with a rough surface having grain
boundaries. The grain boundaries can be in a range of 100 nm to 50
.mu.m. Other suitable ranges include 200 nm to 40 .mu.m, 300 nm to
30 .mu.m, 400 nm to 20 .mu.m, 500 nm to 10 .mu.m, 600 nm to 5
.mu.m, 700 nm to 100 nm to 500 nm, 150 nm to 400 nm, 200 nm to 300
nm, 1 .mu.m to 50 .mu.m, 5 .mu.m to 40 and 10 .mu.m to 30 .mu.m.
The grain boundaries can be, for example, 100 nm, 200 nm, 300 nm,
400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1 .mu.m, 5 .mu.m,
10 .mu.m, 15 .mu.m, 20 .mu.m, 25 .mu.m, 30 .mu.m, 35 .mu.m, 40
.mu.m, 45 .mu.m, or 50 .mu.m.
[0054] Referring to FIG. 7, the second tip can also be formed to
have an aperture at the apex for delivery of ink material through
the tip. The second tip can be a hollow tip (partially or fully
hollow) or a solid tip. For example, as shown in FIG. 7, the second
tip can include a channel for delivery of an ink material through
the channel.
[0055] In addition to the second tip architectures described above,
the synthesis tip can be any known or suitable synthesis or writing
tip, such as those used with dip pen nanolithography and atomic
force microscopy. For example, the synthesis tip can be a high
temperature tip (as illustrated in FIG. 8), an electric field
controlled tip (as illustrated in FIG. 9), a catalyst tipped tip,
an aperture evaporator tip (as illustrated in FIG. 7), an atomic
force microscopy tip, an elastomeric gel tip, or a polymer tip. The
synthesis tip can also be used, for example, for dip pen
nanolithography (DPN). To load the synthesis tip with ink for DPN,
a nanowell can be provided having the ink therein. The nanowell can
further include a resting surface for the non-synthesis tip, such
that the non-synthesis tip is not loaded with ink, while the
synthesis tip remains aligned with the nanowell. The non-synthesis
tip can be placed on the resting surface and a force can be applied
to the cantilever arm to bend the cantilever arm and displace the
synthesis tip into the nanowell, thereby loading it with ink.
[0056] Alternatively, a substrate mold created during the mold and
transfer process for fabricating the dual tip probes (as is
described in detail herein) can be used as ink wells for loading
the dual tip probe for writing, such as for DPN. As a result of the
fabrication process, the openings of the substrate mold are
inherently aligned with the first and second tips (e.g., in
relative location, shape, dimension, etc.). This relationship can
be particularly advantageous when loading an array of dual tip
probes formed from a substrate mold. One of both of the openings
can be filled with an ink, depending whether one or both of the
first and second tips are to be loaded with ink. The first and
second tips can be aligned with the openings and then the probe can
be lowered toward the substrate mold such that the first and second
tips are at least partially disposed within the first and second
openings to contact an ink contained within at least one of the
openings. This method can be particularly useful for loading only a
single tip of the dual tip probe with the ink, as the mold
inherently separates the tips into the two openings, isolating the
non-synthesis tip from the ink.
[0057] Referring to FIG. 6B, the dual tip probe can further include
stiffness-enhancing ribs disposed on the cantilever arm, for
example between the first and second tips. The ribs preferably
comprise an additional layer of cantilever arm material or other
stiffness-enhancing material disposed on the cantilever arm, for
example in linear fashion. The ribs preferably are disposed on the
same side of the cantilever arm from which the tips project, and in
the alternative or in addition can be disposed on the opposite side
of the cantilever arm. The ribs can be oriented between the first
and second tips parallel with respect to the major axis of the
cantilever arm, perpendicular with respect to the major axis of the
cantilever arm, or any range in between, preferably parallel with
respect to the major axis of the cantilever arm which has a length
greater than its width (see FIG. 5). The ribs can have a length,
for example, in a range of 1 .mu.m to 10 .mu.m. Other suitable
lengths include, for example, about 1, 2, 3, 4, 5, 6, 7, 8, 9, and
10 .mu.m. The ribs can have a width, for example, in a range of 0.1
.mu.m to 5 .mu.m, and a thickness, for example, of 0.1 .mu.m to 5
.mu.m. Other suitable widths include, for example, about 0.1, 0.5,
1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, and 5 .mu.m. Other suitable
thickness include, for example, about 0.1, 0.5, 1, 1.5, 2, 2.5, 3,
3.5, 4, 4.5, and 5 .mu.m. The ribs can have any suitable
cross-sectional shape, including, for example, linear, circular,
rectangular, triangular, and semi-circular.
[0058] The distance between the apex of the second tip and the
substrate can be controlled using the dual tip design. For example,
the relationship between the applied force on the cantilever arm
and the distance between the second tip and the substrate of a dual
tip probe can be calibrated using a force-distance curve as shown
in FIGS. 10A and 10B to determine the relationship between the
amount of applied force and the distance between the second tip and
the substrate. A dual tip probe can be calibrated by bringing the
probe towards the surface of the substrate until the first tip is
in contact with the substrate, applying additional force to thereby
bend the cantilever arm between the tips and optionally between the
second tip and base of the cantilever arm to displace the second
tip closer to the substrate. The amount of cantilever bending can
be sensed, for example, from a laser deflection from the cantilever
by means known in the art. The amount of cantilever bending can
also be sensed using one or more strain gauges disposed on the
cantilever arm, for example as illustrated in FIG. 1B. Referring to
FIG. 1C, the change in distance between the second tip and the
substrate can be calculated based on the amount of bending of the
cantilever (.DELTA.z) and the dimensions of the dual tip probe,
using the following relationship:
.DELTA. h 2 = .DELTA. z ( 1 - L 2 L 1 ) ##EQU00001##
wherein .DELTA.h.sub.2, .DELTA.z, L.sub.1, and L.sub.2 are defined
as illustrated in FIG. 1C.
[0059] With reference to FIGS. 10A and 10B, the knee in the
approach curve followed by a sharp rise in force is indicative of
contact. The sharp dip is associated with attractive capillary
forces that cause the tip to snap into contact with the substrate,
and the rise in force indicates the increased force necessary to
push the second tip towards the substrate after contact has been
made by the first tip. Generally, the flat portion of the curve
indicates that the neither tip has contacted the substrate. The
relationship between applied force and the distance between the
second tip and the substrate can be determined from the
force-distance curves using the amount of cantilever bending and
the difference in thickness between the first and second tips.
[0060] Once the force-distance relationship is known, the distance
or gap between the second tip and the substrate can be modulated,
e.g. during a writing process, by varying the applied force. The
distance between the second tip and the substrate can affect the
writing method and can be used to vary dimensions of the
nanostructure synthesized. For example, when a second tip is a
thermal tip or an electric field controlled tip, the distance
between the second tip and the substrate can affect the thermal or
electric field gradient between the substrate and the second tip.
Changes in the gradient can be, for example, used to alter
dimensions of the synthesized nano structures.
[0061] Referring to FIG. 11, the behavior of the thermal or
electric field gradient can be modeled using, for example, ANSYS
finite element analysis and computational fluid dynamics software
known in the art. FIGS. 11A-11C illustrate a simulation of
localized electric field around a tip as a function of the distance
between the tip and the substrate. As illustrated in FIGS. 11B and
11C, when the gap is less than 100 nm, the electrical field is
generally focused inside a 200 nm area. As illustrated in FIG. 11A,
when the gap is about 200 nm, the field begins to diffuse.
Preferably, the gap is less than about 100 nm in order to deposit
materials and make nanometer dimensioned structures on the
substrate. FIG. 11D illustrates temperature diffusion from the tip.
A high temperature zone (about 600.degree. C.) is localized around
the tip. The temperature drops more rapidly with increasing
distance away from the tip. Referring to FIG. 12A, the change of
electric field intensity with distance from the substrate (i.e.
gap) under a fixed voltage bias (20V) is illustrated. FIG. 12B
illustrates the voltage bias required to obtain an electric field
intensity of 5.times.10.sup.9 V/m for a given gap size.
[0062] Referring to FIGS. 13A-13D, the gradient can also be varied,
for example, by changing the tip shape. FIG. 13 illustrates the
effect of various tip surfaces, including a nanorod tip having a
smooth surface and a 50 nm radius, a pyramid tip having a smooth
surface spherical end having a 50 nm radius, a nanorod tip having a
rough surface with 10 nm grain diameter, and a pyramid tip having a
spherical end with a rough surface and a 10 nm grain diameter. The
graphs of FIGS. 13A-13D were generated by contacting each of the
tips with a 7 nm thick SiO.sub.2 layer and applying a voltage bias
of 16V. As shown in FIGS. 13A-13D, the rough surface generally
resulted in increased electric field intensities with the rough
nanorod tip having the highest electric field intensity. Other
parameter modifications such as the amount of the applied field
and/or the temperature can be used to modify the gradient.
[0063] The dual tip probe can further include means for adjusting
the stiffness of the probe. Referring to FIGS. 14A-14D and as
described elsewhere herein, adjustment of the stiffness can change
the force-distance relationship of the probe. The stiffness of the
probe can be adjusted, for example, by varying the dimensions of
the probe, including the lengths L1 and L2 and widths W1 and W2 of
the cantilever arm, the thickness of the cantilever arm, the
thickness of the first and second tips, the distance between tips,
the materials of construction, and the inclusion of one or more
ribs disposed on the cantilever arm, for example between the first
and second tip.
[0064] Referring to FIGS. 14A and 14B, variation of the width of
the first section of the cantilever arm can result in different
force-distance behavior. FIGS. 14A and 14C show in each figure the
force-distance curve during approach (application of force) as the
top line, and during retraction as the bottom line. FIG. 14B shows
the force-distance curve during approach (application of force) as
the bottom line, and during retraction as the top line. In FIG.
14D, the approach and retraction curves substantially overlap,
except that the retract curve shows a sharp decrease in relative
force at a relative distance of about -4 .mu.m. The dimension of
the designs tested (Design 110 and Design 130) in FIGS. 14A and 14B
are included in Table 1 of Example 2. Generally, the width of the
first section of Design 110 was about 20 .mu.m smaller than the
width of the first section of Design 130. As shown in FIG. 14A, for
Design 110, it can be difficult to differentiate on a force
distance curve distinct points when the reading and the synthesis
tips contact the substrate. For initial contact of Design 110
between relative distances -0.75 and -1.00 .mu.m, the stiffness was
about -19.8 nN/.mu.m, while further contact between relative
distances -1.28 and -1.87 .mu.m, the stiffness was about -67.4
nN/.mu.m. As shown in FIG. 14B, for Design 130, variation of the
stiffness of the probe can make it easier to detect distinct points
of contact for the first and second tip. In FIG. 14B, when the
first tip made contact, the stiffness was about 10.8 nN/.mu.m, and
when the second tip made contact the stiffness increased by more
than two fold to -28.1 nN/.mu.m. From this curve, it can e
determined that there is a 200 nm vertical distance between where
the first tip makes contact and the second tip makes contact with
the substrate. As shown in FIGS. 14C and 14D, the addition of ribs
can increase the stiffness value. In fact, the addition of two ribs
can increase the stiffness value of a probe by an order of
magnitude. With such stiff tips it can be difficult to discern
distinct contact points for the first and second tips.
Typical Synthesis Conditions for Nanostructures
[0065] The dual tip probe can be designed to synthesize a variety
of nanostructures and patterns. Synthesis of various nanostructures
using the dual tip probe can be done using synthesis conditions as
are well-known in the art. For example, quantum dots, such as CdS
and CdSe quantum dots are typically synthesized at a temperature in
excess of about 200.degree. C. using, for example, organometallic
precursors in, for example, an inert atmosphere. Quantum dots can
also be synthesized using, for example, an ambient atmosphere.
Carbon nanotubes are typically synthesized at a temperature in
excess of about 550.degree. C., using for example, catalytic
nanoparticles. The catalytic nanoparticles can include, for
example, Fe, Ni, and Co nanoparticles. The carbon nanotubes can be
synthesized in a hydrocarbon environment, such as, for example,
CH.sub.4, C.sub.2H.sub.2, or C.sub.2H.sub.5OH, using a carrier gas,
such as, for example, Ar. Silicon semiconducting nanowires are
typically synthesized at a temperature in excess of 400.degree. C.,
using a catalytic nanoparticle, such as, for example, Au. The
silicon semiconducting nanowires can be synthesized in a SiH.sub.4
and H.sub.2 environment. InP semiconducting nanowires are typically
synthesized at a temperature in a range of 240.degree. C. to
300.degree. C. Catalytic nanoparticles, such as Bi, can be used for
synthesis of the nanowires, in an environment, for example, of
polydecene solutions of In(myristate) and P(SiMe.sub.3). The
synthesis tip of the dual tip probe can be adapted to use the
above-described processing conditions for the formation of various
nanostructures.
Evaporator Synthesis Tip
[0066] Referring to FIG. 9, the synthesis tip can be a variety of
synthesis-type tips, including, for example, an evaporator
synthesis tip with electric field control or thermal capabilities.
The thermal and/or electric field evaporator tip can be used to
form a variety of nanostructures, including, for example,
nanoparticles, nanowires, nanodiscs, quantum dots, nanotubes,
nanopatterns, and combinations thereof. The dimensions of the
nanostructure can be adjusted, for example, by varying the applied
voltage and the pulse width, while placement of the nanostructure
is governed by tip movement.
[0067] The evaporator synthesis tip can be used, for example, for
direct metal deposition onto a substrate. Direct metal deposition
can be useful for formation of nanowires and carbon nanotube
catalysis, plasmonic structures, and in circuitry repair.
Field-induced deposition from an electric field controlled
synthesis tip enables control of the feature size by varying pulse
width and pulse bias voltage. Metal evaporation can occur, for
example, under negative bias, with voltages, for example, in a
range of -8 V to -100 V. Other suitable voltages include, for
example, -10 V to -90 V, -20 V to -80 V, -30 V to -70 V, and -40 V
to -60V, The voltage can be for example, about -8, -9, -10, -15,
-20, -25, -30, -35, -40, -45, -50, -55, -60, -65, -70, -75, -80,
-85, -90, -95, and -100 V.
[0068] The evaporator tip can also be used, for example, to
synthesize semiconducting nanowires and carbon nanotubes (CNT).
Metallic precursor for nanowire and CNT growth can be delivered to
a surface using field induced evaporation from the evaporator tip.
The as-deposited precursor can then be exposed to a gaseous
environment and heated to induce growth the nanowires and/or carbon
nanotubes.
[0069] For example, gold nanoparticles can be used as a catalyst
for epitaxial growth of semiconducting silicon nanowires. Gold
nanoparticles can be deposited onto the synthesis tip and then
transferred to the substrate using field induced evaporation. A Cr
layer can be first deposited onto the synthesis tip as an adhesion
layer. The Cr layer can have a thickness, for example, in a range
of 5 nm to 50 nm. Other suitable thickness include, for example
from 10 nm to 40 nm, 15 nm to 35 nm, 20 nm to 30 nm. The Cr layer
thickness can be, for example, about 5, 6, 7, 8, 9, 10, 15, 20, 25,
30, 35, 40, 45, or 50 nm. The gold layer can have a thickness, for
example, in a range of 50 nm to 500 nm. Other suitable thicknesses
include, for example, from 60 nm to 400 nm, 70 nm to 300 nm, 80 nm
to 200 nm, and 100 nm to 200 nm. The gold layer can have a
thickness, for example, of about 50, 55, 60, 65, 70, 75, 80, 85,
90, 95, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210,
220, 230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340,
350, 360, 370, 380, 390, 400, 410, 420, 430, 440, 450, 460, 470,
480, 490, or 500 nm. The gold nanoparticles can be deposited onto
the substrate using the dual tip probe. Gold nanoparticles can be
deposited, for example, by applying a bias to the tip. The bias can
be in a range, for example, of 8 V to 100 V. Other suitable
voltages include, for example, 10 V to 90 V, 20 V to 80 V, 30 V to
70 V, and 40 V to 60V, The voltage can be for example, about 8, 9,
10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90,
95, and 100 V. The bias can also be applied in short pulses to the
tip. For example, pulses in a range of 1 to 100 ms can be used.
Other suitable pulse times include, for example, 5 ms to 80 ms, 10
ms 70 ms, 20 ms to 60 ms, and 30 ms to 50 ms. The pulse time can be
for example, about 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55,
60, 65, 70, 75, 80, 85, 90, 95, or 100 ms. The pulses can be
controlled using, for example LABVIEW software (National
Instruments, Austin, Tex.) that operates a pulse generator through
a GPIB interface. Upon introduction of a gas, such as silane gas,
for example, formation of the nanowires can proceed through the
vapor-liquid-solid mechanism. The diameter of the nanowire can
depend upon the size of the gold nanoparticle precursor.
[0070] Referring to FIG. 7, the evaporator tip can further include
an aperture formed in the writing portion of the tip (e.g., at the
apex of the tip), through which the writing material can be
deposited. The tip can be, for example, a hollow tip and the
writing material can be contained within the tip and can be caused
to exit the tip upon application of an applied voltage. The rate of
deposition can depend upon the size of the orifice and the
magnitude and duration of the applied field.
[0071] The dual tip design allows for the synthesis through pulsed
evaporation in a non-contact mode. The first tip can operate in
contact mode to provide in situ characterization of the surface
and/or the structures formed, while the evaporator synthesis tip
(i.e. the second tip) remains disposed above the substrate. As one
advantage, this architecture decreases or avoids the consumption,
wear, and change in morphology or dimension of the synthesis tip,
which can have one or more benefits such as improving resolution,
feature size control, and reproducibility. This architecture can
also allow for the extension of pulsed scanning evaporation
deposition of non-conducting precursors that have lower vapor
pressure than that of the tip metal, such as, for example,
stoichiometric solid precursors including bulk CdSe and CdS solids,
and decomposable precursors including CoCl.sub.2, FeCl.sub.2, and
NiCl.sub.2.
Thermal Catalyst-Tipped Synthesis Tip
[0072] Referring to FIG. 8, the synthesis tip can be, for example,
a heated tip. The heated tip can further include a catalyst, such
as, for example, catalytic nanoparticles, disposed on the tip. The
catalytic nanoparticles can include, for example, Fe, Ni, Co, Au,
and Bi nanoparticles. The tip can be heated, for example, by
including a resistive heater on the second tip. Thermal tips can
heat to temperatures, for example, in a range of 100.degree. C. to
700.degree. C. Other suitable temperatures include, for example,
150.degree. C. to 600.degree. C., 200.degree. C. to 500.degree. C.,
and 300.degree. C. to 400.degree. C. The temperature can be, for
example, 100.degree. C., 150.degree. C., 200.degree. C.,
250.degree. C., 300.degree. C., 350.degree. C., 400.degree. C.,
450.degree. C., 500.degree. C., 550.degree. C., 600.degree. C.,
650.degree. C., or 700.degree. C. As shown in FIG. 8D, the highest
temperatures are localized on and around the tip. FIGS. 8D, 8E, and
8G further illustrate that the increased temperature is localized
around the tip, with the temperature decreasing more rapidly with
increasing distance from the tip. It can be desirable that the tips
be able to heat up quickly. For example, it can be desirable to
have tips that reach in excess of about 300.degree. C. in about 15
seconds. The thermal tip can behave as a nanoscale evaporator that
deposits nanoscale quantities of metal in a similar fashion as a
macroscopic thermal metal evaporator. The thermal tips can also
behave as a chemical vapor deposition (CVD) system whereby
nanowires and carbon nanotubes are grown directly from the tip.
[0073] The resistive heater can be formed by patterning metal wires
onto the second probe, which can be for example a silicon nitride
probe. The metal wires can include, for example, a primary
conductor, such as Au wires, a diffusion barrier, such Pt wires,
and an adhesive, such as Cr. The resistive heater can be wire
bonded onto the tip. The temperature of the tip can be determined
using the following relationship by applying a bias across the
restive heater:
R(T)=R.sub.0(1+.alpha.T)
[0074] wherein R(T) is the resistant at temperature T, R.sub.0 is
the resistance at a reference temperature (i.e. room temperature),
and .alpha. is the temperature coefficient of the resistance. The
resistance increases as the applied power increases. Preferably,
the tips can be heated to a temperature in a range of 100.degree.
C. to 700.degree. C.
[0075] Referring to FIG. 15, a heated catalyst-tipped tip can be
used, for example, for formation of CNTs. An increase in
temperature of the catalyst-tipped synthesis tip can be used to
initiate growth of the CNT. The temperate can be increased to be in
a range of 200.degree. C. to 700.degree. C. Other suitable
temperatures include, for example, 200.degree. C. to 600.degree.
C., 250.degree. C. to 500.degree. C., and 300.degree. C. to
400.degree. C. The temperature can be, for example, 200.degree. C.,
250.degree. C., 300.degree. C., 350.degree. C., 400.degree. C.,
450.degree. C., 500.degree. C., 550.degree. C., 600.degree. C.,
650.degree. C., or 700.degree. C. A subsequent temperature drop at
the tip can be used to terminate growth. Thus, the length of a CNT
can be controlled by controlling the temperature of the tip.
Further, the localized temperature control at the tip will minimize
competing thermal decomposition of the catalyst and outgassing of
the system. As illustrated in FIG. 11D, the temperature can
substantially drop at increasing distances away from the tip, which
can allow for localization of the CNT growth at the tip. Thus, CNT
growth can be localized to the tip even when other portions of the
probe are coated with the catalyst. The position of the CNT on the
substrate can be controlled by the movement of the tip on the
substrate. CNT growth can also be initiated by a catalyst-tipped
tip without heat control by heating the substrate to a temperature
in a range of 200.degree. C. to 700.degree. C. Other suitable
temperatures include, for example, 200.degree. C. to 600.degree.
C., 250.degree. C. to 500.degree. C., and 300.degree. C. to
400.degree. C. The temperature can be, for example, 200.degree. C.,
250.degree. C., 300.degree. C., 350.degree. C., 400.degree. C.,
450.degree. C., 500.degree. C., 550.degree. C., 600.degree. C.,
650.degree. C., or 700.degree. C.
Nanostructures Formed in Nanoreactor Wells
[0076] Referring to FIG. 16, a dual tip probe having any of a
variety of synthesis type tips as discussed above can be used to
form nanostructures in a nanoreactor well. Nanoreactor wells can be
synthesized using a variety of known techniques, such as self
assembly, for example via phase separation, of copolymers,
electro-beam lithography, and anodically oxidized aluminum
templates, which results in a series of different nanoreactors,
each with unique, tailorable properties. Referring to FIG. 16B-16D,
nanowells were formed by phase separation of immiscible polymers,
electron-beam lithography, and oxidation of anodic aluminum oxide,
respectively. A probe, such as the dual tip probe, can then be used
to deposit a material into the nanoreactor wells to form, for
example, quantum dots, nanowires, and carbon nanotubes.
Method of Making the Dual Tip Probe
[0077] Referring to FIGS. 17 and 18, the probes having a dual tip
structure can be formed using, for example, a mold and transfer
process. The mold and transfer process utilizes a substrate as a
template for probe formation. The substrate can be, for example, a
silicon wafer. The substrate can have a thickness in a range of 50
to 1000 .mu.m. Other suitable thickness include from 60 .mu.m to
900 .mu.m, 80 .mu.m to 800 .mu.m, 100 .mu.m to 600 .mu.m, 200 .mu.m
to 500 .mu.m, and 300 .mu.m to 400 .mu.m. The substrate can have a
thickness, for example, of about 50, 100, 150, 200, 250, 300, 350,
400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, or 1000
.mu.m. The substrate can be precleaned, for example, by rinsing
with a solvent, preferably an organic solvent (e.g., acetone,
methanol, isopropyl alcohol, or any combination thereof).
[0078] First and second cavities are formed in the substrate. The
cavities can be formed, for example, by anisotropically etching the
substrate. The substrate can be etched using a mask patterned with
two openings defining the first and second cavities. The mask can
be formed, for example, by depositing a mask layer onto the
substrate and patterning the mask to form the two openings. The
mask layer can be, for example, a silicon oxide layer. The mask
layer can have a thickness in a range of 1000 .ANG. to 10000 .ANG..
Other suitable thicknesses include, for example 1100 .ANG. to 9000
.ANG., 1200 .ANG. to 8000 .ANG., 1400 .ANG. to 7000 .ANG., 1600
.ANG. to 6000 .ANG., 1800 .ANG. to 8000 .ANG., 2000 .ANG. to 6000
.ANG., and 4000 .ANG. to 5000 .ANG.. The mask layer can have a
thickness for example, of about 1000, 1500, 2000, 2500, 3000, 3500,
4000, 4500, 5000, 5500, 6000, 6500, 7000, 7500, 8000, 8500, 9000,
9500, and 10000 .ANG.. The mask layer can be, for example,
thermally grown on the substrate. For example, a 5000 .ANG. silicon
oxide layer can be thermally grown on a silicon wafer at
1110.degree. C. for about 13 hours to form the mask layer. The
openings can be formed, for example, using electron-beam
lithography or can be etched using, for example, wet chemical
etching. The wet chemical etching can use HF as the etching
solution. The size of the openings can be correlated to the depth
of the subsequently formed cavities. For example, if the first
opening is formed larger than the second opening, a first cavity
that is deeper than the second cavity will be subsequently formed.
This can be further correlated to the thickness (height, as
measured from the base connected to the cantilever arm) of the
subsequently formed tips. Accordingly, the thickness difference
between the first and second tips can be controlled by controlling
dimensions of the openings formed in the mask.
[0079] The cavities can then be etched into the substrate, for
example, by a wet chemical etching process, using, for example KOH.
The cavities have a shape corresponding to the desired tip shape.
For example, the cavities can have a pyramidal shape where it is
desired to form pyramidal tips. The mask can then be removed, using
for example, BOE. The cavities can also be formed, for example, by
first patterning square openings onto a substrate, for example an
oxidized <100> silicon wafer, and then immersing the
substrate can then be immersed in an etch solution, such as KOH, to
anisotropically etch pyramidal pits into the substrate. Etching is
generally terminated at <111> for a <100> silicon
wafer, which can prevent over-etching of the substrate.
[0080] A sacrificial layer can then be formed on the substrate
including the cavities. The sacrificial layer can be for formed,
for example, by oxidation, chemical vapor deposition, low pressure
chemical vapor deposition, or physical vapor deposition. The
sacrificial layer can be formed, for example, from metals such as,
copper, permalloy, tungsten, titanium, aluminum, silver, gold,
oxides, such as silicon oxide, silicon dioxide, silicon oxynitride,
and zinc oxide, nitrides, such as silicon nitride and titanium
nitride, polymers, such as poly(dimethylsiloxane) (PDMS),
polyimide, parylene, elastomers, such as silicone and rubber, and
photoresists such as SU-8. The sacrificial layer can have a
thickness, for example, in a range of 500 .ANG. to 5000 .ANG..
Other suitable thicknesses include, for example, 600 .ANG. to 4000
.ANG., 700 .ANG. to 3000 .ANG., 800 .ANG. to 2000 .ANG., and 900
.ANG. to 1000 .ANG.. The sacrificial layer can have a thickness,
for example, of about 500, 600, 700, 800, 900, 1000, 1500, 2000,
2500, 3000, 3500, 4000, 4500, or 5000 .ANG..
[0081] A first tip layer for forming the first tip can then be
deposited onto the sacrificial layer. The first tip layer is then
patterned to remove at least a portion of the first tip layer
disposed outside the first cavity. The first tip layer can also be
patterned such that only a portion of the first tip layer disposed
in the first cavity remains to form the first tip. The first tip
layer can, for example, be photolithographically patterned and
chemically etched to form the first tip. If the first tip is an
imaging tip, then the first tip layer preferably includes, for
example, a dielectric layer, such as a silicon nitride layer or a
silicon dioxide layer, which can electrically isolate the tip from
the substrate during imaging. Other suitable materials for imaging
tips are known in the art and are described elsewhere herein. The
first tip layer can be etched, for example, using a plasma
etching.
[0082] A second tip layer for forming the second tip can be
deposited onto the sacrificial layer. The second tip layer can be
deposited, for example, by low pressure chemical vapor deposition
The second tip layer can then be patterned to remove at least a
portion of the second tip layer disposed outside the second cavity.
The second tip layer can also be patterned such that only a portion
of the second tip layer disposed in the second cavity remains to
form the second tip. The second tip layer can, for example, be
photolithographically patterned and chemically etched to form the
second tip. If the second tip is a writing tip, then the second tip
layer can be, for example, a conductor, such as a doped polysilicon
layer or a metal layer, such as gold or aluminum. Other suitable
materials for the writing and synthesis tips are known in the art
and are described elsewhere herein.
[0083] A cantilever arm layer can be deposited on the sacrificial
layer including the first and second layers. The cantilever arm
layer can be optionally patterned to remove at least a portion of
the cantilever arm layer disposed on the patterned first and second
tip layers. Alternatively, the first and/or second tip layers can
be patterned such that one or both of the first and/or second tip
layers extend outside of the first and/or second cavities to form
the cantilever arm. The tip layers and the cantilever arm layer can
be deposited so that at least a portion of the layers overlap. A
handle wafer can then be attached to the cantilever arm layer. The
sacrificial layer can then be selectively etched to remove the dual
tip probe from the substrate.
[0084] Where a thermal or electric field controlled dual tip probe
is desired, a conductive material can be deposited in or around the
second tip. For example, an electrical biasing layer, an electrical
insulating layer, and an electrical conductor or heater can be
sequentially deposited on top of the second tip layer. One or more
of the electrical biasing layer, the electrical insulating layer,
and the electrical conductor layer can optionally be formed to
extend within at least a portion of the second tip opening. These
layers can be used, for example, to form the cantilever arm. A
handle can be attached to the cantilever arm and wires can be
attached to the handle, for example, to provide an electrical feed
through. A thermal tip can also be formed by attaching a resistive
heater as described above to the one or both of the first and
second tips, using for example, a wire bonding method.
[0085] Referring to FIG. 18, the method can further include forming
an aperture in the second tip in order to form the second tip as an
aperture evaporation tip. The size of the aperture can be
controlled to control the dimensions of the evaporated material
deposited onto a substrate. The aperture can have a width in a
range of 5 nm to 200 nm. Other suitable widths include, for
example, 10 nm to 150 nm, 20 nm to 100 nm, and 40 nm to 50 nm. The
aperture can have a width for example, of about 5, 10, 15, 20, 25,
30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110,
120, 130, 140, 150, 160, 170, 180, 190, or 200 nm. The aperture can
be formed, for example, using focused ion beam (FIB) etching or
electrochemical plating. FIB etching can be used to etch patterns
with a line resolution in a range of 30 nm to 50 nm. See Wang et
al., 87 Appl. Phys. Lett. 054102 (2005). The size of the aperture
can be further reduced (i.e., the tip built up around the
aperture), if necessary, using, for example, selective thermal
oxidation or material deposition.
[0086] The mold and transfer method can provide one or more
advantages, including, for example, tip uniformity within an array
of tips, a variety of tip and cantilever material combinations can
be used, various tip and cantilever materials can be integrated
into the same array, providing multiplexed functions, uniform tip
sharpness, a master substrate that can be reused thereby reducing
cost of fabrication over time, a master substrate that can be used
as ink wells for loading one or more tips with an ink or other
material, and high fabrication yield and uniformity. The shape and
dimensions of the tip can be controlled and pre-determined in the
mold and transfer process by photolithography and self-limiting
etching process. The sharpness of the tip can be controlled and
determined by the etched cavities in the substrate and the
subsequently deposited tip layers. The conventional mold and
transfer process has been used to realize a million pen probe array
(single tipped probes) with 100% yield.
Software Platform for Pulsed Evaporation Synthesis
[0087] A manual application of voltage pulses to the evaporator
synthesis tip and human-assisted repetition of experiments can
introduce error and become prone to human errors. A software
platform can be used to automate and enable precise experiments of
evaporation from an electric-field controlled tip using voltage
pulses. The software can enable high-precision control of the
movement of the evaporator synthesis tip and application of voltage
pulses on the evaporator synthesis tip. The software can also
enable the monitoring of the current flowing through the
tip/surface interface.
[0088] Referring to FIG. 27, a system for scanning pulsed
evaporation of metals can use various hardware including a voltage
pulse generator, a current preamplifier, an oscilloscope, a data
acquisition board (DAQ), a computer (general purpose processor or
logic chip, or a general purpose computer), and communication
interfaces associated therewith for communication between the
components.
[0089] The software can include a set of two processes running in
two different physical locations. Process 1 is a program. The
program source code can be programmed into, for example, Labview
(National Instruments), a graphical interface programming language.
The Process 2 can be run, for example, over an AFM control
software, and the source code can be programmed, for example, using
Nanoscript functions provided by the AFM manufacturer and general C
programming language. The two processes wait and exchange signals
with the other process for timing and scheduling the whole progress
of hardware (HW) control.
[0090] Process 1 controls the data acquisition board (DAQ) to
acquire the READY signal (voltage pulse) from the AFM controller.
Process 1 further controls the data acquisition board (DAQ) to send
out an OK signal to the AFM controller, so the AFM controller stops
waiting and proceeds with the next instructions. Process 1 also
controls the function generator, and sets the amplitude, pulse
width, pulse period, pulse number, and pulse shooting.
[0091] Process 2 controls the vertical and lateral transitional
movements of the probe. Process 2 control can be designed, for
example, to control the vertical and lateral transitional movements
of a dual tip probe having first and second tips as described
above. Process 2 control can further control the amount of
cantilever bending to operate the first tip in contact mode while
simultaneous operating the second tip in non-contact mode and
modulating the distance between the second tip and the substrate.
Process 2 also controls a Signal Access Module (SAM) to send out
the READY signal to Process 1. Process 2 controls the SAM to read
in the OK signal from Process 1 to stop waiting and proceed with
the next instructions.
[0092] The following exemplary flow description describes the role
and the signaling sequence between different units of the system
during two succeeding "feedback on" events. The probe controlled in
the flow description can be any known probe, for example, an AFM
probe or the above-described dual tip probe. The two separate
process (Process 1 and 2) can run on different computers (PC1 and
PC2), and can exchange signals to schedule the events.
Flow Description (Between Two Successive Feedback-Ons)
TABLE-US-00001 [0093] 1 The system gets into feedback. The tip gets
in contact with the substrate, and the feedback loop maintains a
constant set point value. 2 The process 2 sends a ready signal to
process 1 and enters into waiting mode, where it waits for an OK
signal form process 1. 3 Process 1, knowing that the tip is in
feedback with the substrate, makes the function generator apply the
pulses with predefined amplitude, period and width. 4 Process 1,
after having finished with applying pulses, sends an OK signal to
the AFM controller. 5 The AFM controller, having received an OK
signal from process 2, turns off feedback and lifts the probe from
the substrate, and moves the probe to the next position (go there).
6 After reaching the next position, the AFM lowers the tip down 90%
the distance it lifted before, and from there turns on the feedback
loop. The probe makes a soft approach, eventually reaching a
constant set point controlled by the feedback loop.
Synthesis of Nanostructures Using Block Copolymer Template
[0094] Referring to FIGS. 19 and 20, nanostructures can be formed
using for example a block copolymer as a template. The template can
be formed by patterning a block copolymer on a substrate using any
known writing method such as dip pen nanolithography. The dual tip
probe can be used, for example, to pattern the block copolymer.
Once patterned on the substrate, the block copolymer can phase
separate to form the template having at least one smaller dimension
than the initially formed pattern. For example, the block copolymer
can be patterned and then allowed to phase separate to shrink the
size of the templated pattern. The block copolymer can be, for
example, polystyrene-b-poly(2-vinylpyridine),
polystyrene-b-polyethylene oxide,
polystyrene-b-poly(methylmethacrylate),
polystyrene-b-poly(4-vinylpyridine), or
polystyrene-b-poly(methylmethacrylate). The template can be loaded
with the nanostructure precursor material, such as, for example, a
metal, such as gold aluminum, silver, platinum, palladium, or
silicon. The block copolymer can then be removed, for example, by
etching the polymer. For example, the block copolymer can be
removed using oxygen or hydrogen plasma etching or by exposure to
UV light. Referring to FIGS. 19 and 20, the block copolymer can be
patterned to form a nanoarray or nanostructures, such as an array
of nanoparticles or a nanowire. The block copolymer template method
can be used to form nanostructures having narrower widths or
diameters in a range of 1 nm to 50 nm. Other suitable ranges
include for example, from 5 nm to 40 nm, 10 nm to 30 nm, and 15 nm
to 20 nm. The width or diameter of the nanostructure can be, for
example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,
18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34,
35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49 or 50
nm. Other nanostructures that can be formed using the block
copolymer template include, for example, quantum dots, nanodiscs,
and nanotubes.
EXAMPLES
[0095] The following examples are provided for illustration and are
not intended to limit the scope of the invention.
Example 1
Method of Forming a Dual Tip Probe
[0096] Referring to FIG. 21, a dual tip probe was formed by a mold
and transfer process. Silicon wafers were rinsed using acetone,
methanol and isopropyl alcohol. Then, a 5000 .ANG. silicon oxide
layer was thermally grown on the silicon wafers at 1100.degree. C.
for 13 hrs 25 min. After the openings were etched using HF for 6
mins into the silicon wafers, the dual-tip cavities were etched
using KOH. Optical microscopy was used to verify that the tip
etching was complete. Subsequently, the oxide layer was stripped
using BOE. A 2500 .ANG. layer of Cu were sputtered onto the
patterned wafer as a sacrificial layer. Then, an 1500 .ANG. layer
of low-stress silicon nitride was deposited using STS plasma
enhanced chemical vapor deposition (PECVD), and the first silicon
nitride first (reader) tip was patterned and etched.
[0097] A 500 nm Au layer was lifted off as the seed layer for the
dual-tip cantilever and holder. A 10 .mu.m thick AZ4260 layer was
patterned as the mold and 1 .mu.m NiFe was electroplated inside the
cantilever and probe holder area following 100 nm Au deposition as
the adhesion layer inside the probe holder area. The 500 .mu.m
thick SU8 holder was spun on the wafer at 500 rpm for 30 s. The
thickness of the spun SU-8 2075 was about 500 .mu.m. The coated
wafer was prebaked; the hotplate was ramped up from room
temperature to 105.degree. C. using 150.degree. C./hr ramp and
soaked for 15 hr. Following the prebake, the SU8 coated wafer was
exposed for 2880 mJ. Then, the wafer was post-exposure baked by
ramping the temperature from room temperature to 105.degree. C.
using 150.degree. C./hr. The wafer was soaked at 105.degree. C. for
0.5 hr, and then the temperature was ramped down at 15.degree.
C./hr to room temperature. Finally, the SU8 coated wafer was
developed for 1 hour to complete the fabrication process. The
dual-tip probes were released by immersing the wafers in an aqueous
acetic acid/peroxide solution (Acetic
Acid:H.sub.2O.sub.2:H.sub.2O=1:1:10) for 4 hours.
[0098] Referring to FIG. 22, the cantilevers of the metal dual-tip
probes formed in accordance with the above-described method
demonstrated some bending. Bending of the cantilever can lead to
difficulty aligning a laser on the cantilever. Without intending to
be bound by theory, it is believed that the bending resulted from
the bimetallic structure of the gold seed layer and the NiFe
electroplating layer, which possessed intrinsic strain causing the
cantilever to have unbalanced stress. The unbalanced stress can
also result in unpredictability between the bending of individual
cantilevers in an array.
Example 2
Method of Forming a Dual Tip Probe
[0099] The above-described mold and transfer method was used to
make a dual tip probe, with two modifications to the method: (1) A
Cu sacrificial layer is chosen as the electroplating seed layer so
that the metal cantilever has only a single NiFe layer, and (2) the
second metal probe was protected using a photoresist during the
NiFe electroplating so that the thickness of the second metal probe
was precisely controlled by the thermal evaporation. Referring to
FIG. 23, the bimorph structure of the dual tip probes of Example 1
was replaced by a single layer of silicon nitride, which can aid in
eliminating the bending problem identified with the dual tip probes
of Example 1. Without intending to be bound by theory, it is
believed that the bending problem was not experienced with the dual
tip probes formed by the method of Example 2 because the stress in
the use of only a single NiFe layer in the cantilever is even.
Further, it is believed that the electroplated NiFe area inside the
holder was limited to increase the adhesion of the SU8 holder.
Arrays of holes were designed inside the SU8 holder to release
thermal stress, thereby avoiding the peeling issue. While some
cantilevers exhibited some bending, it is believed that this was a
result of a fast current ramping rate during NiFe electroplating,
which generated heat, causing the NiFe layer to peel off towards
the edges of the prove holder. It is believed that this problem can
be avoided by ramping the electroplating current more slowly.
[0100] Table 1 illustrates the dimensions of various dual tip
probes formed in accordance with the above-described method.
TABLE-US-00002 TABLE 1 Dimension of Dual Tip Probes Formed Using
the Method In Accordance with the Invention. Tip Size Number Design
L1 (.mu.m) L2 (.mu.m) W1 (.mu.m) W2 (.mu.m) (.mu.m) of Ribs 1_1_0
124 40 40 40 5 0 1_2_0 124 40 30 30 5 0 1_3_0 124 40 20 20 5 0
1_4_0 124 40 15 15 5 0 2_1_0 124 40 40 35 5 0 2_2_0 124 40 40 30 5
0 2_3_0 124 40 40 25 5 0 2_4_0 124 40 40 20 5 0 2_5_0 124 40 40 15
5 0 2_1_2 124 40 40 40 8 0 3_1_2 124 40 40 40 11 0 1_1_3 124 40 40
40 5 1 2_1_3 124 40 40 40 5 2 3_1_3 124 40 40 40 5 3 4_1_3 124 40
40 40 5 4 1_1_1 124 40 40 40 5 1 2_1_1 124 40 40 40 5 2 3_1_1 124
40 40 40 5 1 4_1_1 124 40 40 40 5 2
Example 3
Method of Making a Thermal Probe
[0101] Referring to FIG. 8, the dual tips probes can include a
thermal tip as the synthesis tip. The thermal tip was formed by
attaching a resistive heater to the synthesis tip. The resistive
heater was formed by patterning metal wires onto the silicon
nitride tip. The metal wires included a 10 nm Cr wire, a 30 nm Pt
wire, and a 400 nm Au wire. The Au was used as the primary
conductor, the Pt was used as a diffusion barrier, and the Cr was
used as an adhesive layer. Referring to FIG. 8A, the metal wires
were wire bonded to the tip using the standard technique. The probe
was processed through wafer bonding, tetramethylammonium hydroxide
(TMAH) etching, and oxide etching, leaving a 100 mm diameter
silicon frame with all the die locations attached securely, but
nevertheless able to be diced quickly, and with the cantilevers all
free and clear. Optical microscopy confirmed that the tips were
well formed, and the cantilevers without gold were flat and
uniform. Cantilevers with metal exhibited a stress mismatch between
the Au and silicon nitride. Die yield was about 70% to about 80% on
the first wafer.
Example 4
Force-Distance Curve Calibration of Dual Tip Probe
[0102] Referring to FIGS. 14A-D, as a control, force-distance
measurements of a single tip (not shown), commercially available
probe (Pacific Nanotechnology) were taken. It was observed that the
knee in the approach curve followed by a sharp rise in force is
indicative of contact. The sharp dip is associated with attractive
capillary forces that cause the tip to "snap" into contact with the
substrate, and the rise indicates the increased force necessary to
push the tip into the surface after contact has been made.
Generally, the flat line of the curve higher on the relative
distance axis indicates that the tip and substrate are not in
contact. This curve was measured multiple times starting from
different z-piezo distances and always the same phenomenon was
observed.
[0103] The spring constants for the silicon nitride dual tip probes
tested was assumed for all measurements to be 0.200 N/m with a
sensitivity of 3.506 mV/nm. This estimated spring constant value
was used because the silicon nitride dual tip probes were formed
from masks used for the commercially available NanoInk Active Pen
arrays. For the Active Pen arrays, the reported spring constant of
silicon nitride tips which were 30 .mu.m wide and 150 .mu.m long is
0.180 N/m; these geometric parameters most closely resemble the
dual tip probes tested, which have a maximum width of 40 .mu.m and
maximum length of 164 .mu.m.
[0104] For the dual tip probes with a reflective back layer, a
photodiode was able to detect and measure a sufficient laser
signal. Thus, it was possible to make force-distance measurements
and detect distinct points of contact for both the first and second
tip. The contact of the second tip is indicated by a second knee in
the force distance curve with a concomitant increase in the force
required to further extend the z-piezo. Though in the single tip
force-distance curve there was a flat line for the points where
there was no tip-substrate contact, the dual tip probe does not
generate a straight line when not in contact. Without intending to
be bound by theory, it is believed that this anomalous behavior is
the result of the dual tip cantilever bending beyond the range of
the z-piezo (13 .mu.m). For bending that exceeds this amount, there
may be a laser signal detected that is not real and not
representative of tip-substrate contact.
[0105] Referring to FIGS. 14A-14D, a comparison of how dimensions
affect the dual tip probe's stiffness was performed using
force-distance curves. Referring to FIGS. 14A and 14B, two dual tip
probes having cantilever arms with different first second widths W1
were compared, designs 110 and 130, the dimensions of which are in
Table 1 above. The two probes had a difference of 20 .mu.m in the
first section widths, with design 110 being wider than design 130.
It was difficult to observe distinct points for design 110 when the
writing and synthesis tips made contact with the substrate. For
initial contact of dual tip probe design 110 between relative
distances of -0.75 and -1.00 .mu.m, the stiffness was -19.8
nN/.mu.m, while further contact between the relative distances of
-1.28 and -1.86 .mu.m the stiffness was -67.4 nN/.mu.m. For dual
tip design 130, when the first tip makes contact, the stiffness is
-10.8 nN/.mu.m, as given by the linear slope; when both tips are in
contact, the stiffness increases by more than two-fold to -28.1
nN/.mu.m. The force distance curves show that there is a 200 nm
vertical distance between where the first (imaging) tip makes
contact and the second (synthesis) tip makes contact with the
substrate. It was found that the stiffness values are higher for
dual tip probe with a large first second width (i.e. design
110).
[0106] In another dual tip design comparison, the effect of ribs
disposed between the first and second tips was examined by
comparing Design 110 and 213. Again, it was difficult to observe
distinct contact points for Design 213 that distinguish the first
tip from the second tip, and only one stiffness value was measured,
-752 nN/.mu.m (FIG. 14B). The addition of two ribs makes the
stiffness values increase by an order of magnitude. Moreover, with
such stiff tips, it was difficult to discern the contact point of
the synthesis tip.
Example 5
Imaging Capabilities of the Dual Tip Probe
[0107] Imaging capabilities of the dual-tip probes were
demonstrated in both contact mode and non-contact mode using a
calibration grid. Images were obtained on a
9.9.times.9.9.times.0.175 .mu.m calibration grid using the dual tip
probe (FIG. 24B), and compared to images obtained by
high-resolution scanning probe tips from NanoProbes, Inc. (Yaphank,
N.Y.) (FIG. 24A). The dual-tip probes were capable of obtaining
high-quality images. The high-resolution probes and the dual-tip
probes obtained the same value for the depth of the wells in the
calibration grid (175 nm). There was a discrepancy, however, in the
wall-to-wall distance. While the high-resolution probes measured a
distance of 9.9 .mu.m, a distance of 10.2 .mu.m was obtained with
the dual-tip probes. Without intending to be bound by theory, it is
believed that this discrepancy is the result of the larger tip
radius of the dual-tips. Importantly, the dual-tip probes obtained
images under high load, indicating that even when large amounts of
force are applied to the cantilever, the writing tip does not
interfere with imaging capabilities.
Example 6
Formation of Nanowires and Carbon Nanotubes Using Field-Induced
Evaporation
[0108] Gold was evaporated using electric field induced evaporation
of gold from a conductive synthesis tip onto a silicon dioxide
surface. Low-resistivity AFM tips were used as the synthesis tip
and were coated with a 5 nm Cr adhesion layer followed by the
thermal deposition of a 100 nm gold layer. Patterns of gold on the
surface were generated by electric-field induced migration of the
gold from the probe tip to the surface. A custom-built platform was
used to induce the deposition of gold onto the surface. Short
electrical pulses (1-100 ms) of 20 V bias were applied to the tip.
The pulses were controlled with custom LABVIEW software (National
Instruments, Austin, Tx) that operated a pulse generator through a
GPIB interface. The probe was mounted onto a Multimode III AFM
platform (Digital Instruments) with a MMTR-TUNA-CH cantilever
holder that isolates the piezo from the electric fields applied to
the probe. Evaporation was induced in contact mode, and pulses were
monitored in real-time with an oscilloscope.
[0109] The resulting patterns of gold nanoparticles were observed
by both AFM topological imaging (FIG. 25) and SEM characterization
(FIG. 26). By applying 1 ms pulses at a 20V bias and rate of 10 Hz,
a square pattern of 10 nanoparticles per line was generated. The
height of the gold particles was measured to be approximately 8 nm
by AFM topographical imaging. The patterns were also seen in the
SEM, and the contrast observed in the images is consistent with
gold on silicon dioxide. Further confirmation that the structures
were gold was established by imaging single particles evaporated
from the probe. Larger particles were generated by applying longer
pulses to the tip. A 10 ms pulse generated gold features 250 nm in
height with distinctive hexagonal shapes and angles of 60.degree..
Because gold (111) has a hexagonal lattice, it is believed that if
the gold forms in a crystalline fashion, the resulting
nanostructures would be hexagonal. Additionally, larger features
contain terraces that are also indicative that single-crystal gold
was formed. This method of fabricating nanostructure by varying
bias, pulse width and pulse period can result in the ability to
determine the position and feature size of nanostructures with
precise control.
[0110] FIG. 3 shows an non-contact (nc) AFM images of Au patterns
on a silicon dioxide surface drawn with this technique using a dual
tip probe. A series of nc-AFM images of dot-shaped Au pattern
before and after multiple depositions are shown in FIGS. 3A-3C,
clearly demonstrating the electric field-induced evaporation onto
the surface. By applying 20 .mu.s at a 12 V bias, a square pattern
with 25 dots was generated (FIG. 3B), compared to the surface
before deposition (FIG. 3A). It was determined that the first
pattern contained an error, namely the omission of a square pattern
of 16 dots between the 25 dot square first pattern. This error was
detected using the non-synthesis probe, and a second square pattern
with 16 dots (FIG. 3C) is drawn between the first pattern with the
same voltage pulse and tip to correct the error. This result
clearly reflects the ability of the dual tip probe to determine the
site-specifically controlled protocol for patterning Au dots.
Importantly, this experiment demonstrates error-correcting ability
of the dual tip probe with nanometer precision.
Example 6
Nanoreactor Well Formation
[0111] Referring to FIGS. 16A-16D, nanowells were fabricated on
silicon dioxide wafers. Referring to FIG. 16C, a nanowell was
formed using electron beam lithography. A 120 nm layer of
poly(methyl methacrylate) (PMMA) photoresist was spin-coated onto a
silicon substrate and fabricated circular well patterns about 50 nm
in diameter using electron beam lithography (EBL). The nanowells
were shown to be highly ordered and uniform by AFM imaging, but the
throughput was slow because EBL is a serial technique.
[0112] Referring to FIG. 16D, electrochemical methods were also
used to fabricate AAO nanowells. In one experiment, 70 nm of
aluminum was evaporated and anodized for 40 seconds in 0.3M oxalic
acid with 40V applied bias. In this process, the choice of acid
determines the pore diameter. Oxalic acid made 30-50 nm diameter
pores. The nanowells were characterized using scanning electron
microscopy (SEM). The bottom side of the AAO in contact with the
silicon substrate formed well-ordered pores, the top side remained
rough and disordered. In order to decrease the surface roughness
and have well-aligned nanowells extended to the top side, a thicker
layer of aluminum (225 nm) was evaporated, a first anodization
(0.3M oxalic acid, 80 seconds, 40V) was completed, and the top
layer was etched (phosphoric and chromic acid, 63.degree. C.,
different times), and added a second anodization step.
[0113] Referring to FIG. 16B, nanotemplates, such as nanowells were
also formed using phase separating polymers like polystyrene (PS)
and polymethyl methacrylate (PMMA). Immiscible polymer phase
separate into well ordered domains that can be removed selectively,
leaving behind well-ordered wells. PMMA cylinders can be aligned
vertically in a PS matrix such that they are perpendicular to the
substrate. By exposing the PS to ultraviolet (UV) light, the
polymer crosslinks and selective removal of PMMA can be achieved
with acetic acid washing. AFM topography images reveal that there
are indeed PMMA nanowells approximately 15 nm in diameter and
height. To render the nanowell surface hydrophilic, timed oxygen
plasma cleaning experiments were performed to make sure PMMA was
not destroyed or removed.
[0114] Following a one minute plasma clean, nanostructure precursor
materials (e.g., silver nitrate, sodium citrate, and sodium
hydroxide) were dropcast onto the wells. These precursors were
exposed to UV light for about 30 minutes form nanoparticles.
Nanoparticles, however, did not form inside the wells. Without
intending to be bound by theory, it is believed that this is most
likely because of the high surface tension of water. By creating
nanowells using several different approaches, nanoarchitectures
with different size, shape and surface chemistry can be
created.
[0115] While the present invention has now been described and
exemplified with some specificity, those skilled in the art will
appreciate the various modifications, including variations,
additions, and omissions that may be made in what has been
described. As one example, while various embodiments have been
described as including a cantilever with two tips, other
embodiments are contemplated to have more than two tips, e.g.,
three, four, or five tips, without limit. Accordingly, it is
intended that these modifications also be encompassed by the
present invention and that the scope of the present invention be
limited solely by the broadest interpretation that lawfully can be
accorded the appended claims.
[0116] All patents, publications and references cited herein are
hereby fully incorporated by reference. In case of conflict between
the present disclosure and incorporated patents, publications and
references, the present disclosure should control.
* * * * *