U.S. patent application number 13/450333 was filed with the patent office on 2012-11-22 for methods utilizing scanning probe microscope tips and products therefor or produced thereby.
This patent application is currently assigned to NORTHWESTERN UNIVERSITY. Invention is credited to Seunghun Hong, Chad A. Mirkin, Richard Piner.
Application Number | 20120295029 13/450333 |
Document ID | / |
Family ID | 26812875 |
Filed Date | 2012-11-22 |
United States Patent
Application |
20120295029 |
Kind Code |
A1 |
Mirkin; Chad A. ; et
al. |
November 22, 2012 |
METHODS UTILIZING SCANNING PROBE MICROSCOPE TIPS AND PRODUCTS
THEREFOR OR PRODUCED THEREBY
Abstract
The invention provides a nanolithographic method, comprising:
(i) providing a substrate; (ii) providing a nanoscopic tip coated
with a patterning compound; (iii) contacting the coated tip with
the substrate so that the patterning compound is applied to the
substrate to produce a desired pattern; and (iv) wherein the
patterning compound is anchored to the substrate.
Inventors: |
Mirkin; Chad A.; (Wilmette,
IL) ; Piner; Richard; (St. Louis, IL) ; Hong;
Seunghun; (Chicago, IL) |
Assignee: |
NORTHWESTERN UNIVERSITY
|
Family ID: |
26812875 |
Appl. No.: |
13/450333 |
Filed: |
April 18, 2012 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11933275 |
Oct 31, 2007 |
8247032 |
|
|
13450333 |
|
|
|
|
10449685 |
Jun 2, 2003 |
7569252 |
|
|
11933275 |
|
|
|
|
09477997 |
Jan 5, 2000 |
6635311 |
|
|
10449685 |
|
|
|
|
60115133 |
Jan 7, 1999 |
|
|
|
60157633 |
Oct 4, 1999 |
|
|
|
Current U.S.
Class: |
427/256 ;
257/E21.002; 427/127; 438/758 |
Current CPC
Class: |
Y10T 436/24 20150115;
G03F 7/165 20130101; G01Q 80/00 20130101; B05D 1/185 20130101; Y10S
977/863 20130101; G03F 7/0002 20130101; Y10S 977/857 20130101; Y10S
977/84 20130101; Y10S 977/877 20130101; Y10T 428/24917 20150115;
B05D 1/26 20130101; B82B 3/00 20130101; B05D 1/007 20130101 |
Class at
Publication: |
427/256 ;
427/127; 438/758; 257/E21.002 |
International
Class: |
B05D 5/00 20060101
B05D005/00; H01L 21/02 20060101 H01L021/02 |
Goverment Interests
[0002] This invention was made with government support under grant
F49620-96-1-055 from the Air Force Office Of Science Research. The
government has rights in the invention.
Claims
1-45. (canceled)
46. A nanolithographic method, comprising: providing a substrate;
providing a nanoscopic tip coated with a patterning compound;
contacting the coated tip with the substrate so that the patterning
compound is applied to the substrate to produce a desired pattern;
and wherein the patterning compound is anchored to the
substrate.
47. The nanolithographic method of claim 46, wherein the desired
pattern has at least one lateral dimension of 1 micron or less.
48. The nanolithographic method of claim 46, wherein the patterning
compound is anchored to the substrate by chemisorption.
49. The nanolithographic method of claim 46, wherein the patterning
compound is anchored to the substrate by covalent bonds.
50. The nanolithographic method of claim 46, further comprising
applying a second compound to the desired pattern, wherein the
second compound chemisorbs or covalently binds to the patterning
compound.
51. The nanolithographic method of claim 46, wherein the substrate
is metal or metal oxide.
52. The nanolithographic method of claim 46, wherein the substrate
is a semiconductor.
53. The nanolithographic method of claim 46, wherein the substrate
magnetic.
54. The nanolithographic method of claim 46, wherein the substrate
is Si, SiO2 or glass.
55. The nanolithographic method of claim 46, wherein the patterning
compound comprises a nucleic acid or oligonucleotide.
56. The nanolithographic method of claim 46, wherein the patterning
compound comprises a protein or peptide.
57. The nanolithographic method of claim 46, wherein the patterning
compound comprises an sulfur-containing compound.
58. The nanolithographic method of claim 46, wherein the patterning
compound comprises an silane compound.
59. The nanolithographic method of claim 46, wherein the patterning
compound comprises an carboxylic acid, an amino compound, or an
alcohol.
60. The nanolithographic method of claim 46, wherein the patterning
compound is coated onto the tip by dipping the tip into a
composition comprising the patterning compound.
61. The nanolithographic method of claim 46, wherein the method is
carried out with a plurality of tips.
62. The nanolithographic method of claim 46, wherein the desired
pattern comprises an array of dots and/or lines.
63. The nanolithographic method of claim 46, wherein the tip is a
wet tip coated with the patterning compound and a solvent.
64. A nanolithographic method, comprising: providing a substrate;
providing a nanoscopic tip; coating the tip with a patterning
compound; and contacting the coated tip with the substrate so that
the patterning compound is applied to the substrate to produce a
desired pattern having at least one lateral dimension of 1 micron
or less.
65. A nanolithographic method, comprising: providing a substrate;
providing a plurality of nanoscopic tips; coating the tips with a
patterning compound; and contacting the coated tips with the
substrate so that the patterning compound is applied to the
substrate to produce an array of dots or lines having at least one
lateral dimension of 1 micron or less.
Description
[0001] This application claims benefit of provisional application
60/115,133, filed Jan. 7, 1999, and provisional application
60/157,633, filed Oct. 4, 1999, the complete disclosures of which
are incorporated herein by reference.
FIELD OF THE INVENTION
[0003] This invention relates to methods of microfabrication and
nanofabrication. The invention also relates to methods of
performing atomic force microscope imaging.
BACKGROUND OF THE INVENTION
[0004] Lithographic methods are at the heart of modern day
microfabrication, nanotechnology and molecular electronics. These
methods often rely on patterning a resistive film, followed by a
chemical etch of the substrate.
[0005] Dip pen technology, where ink on a sharp object is
transported to a paper substrate by capillary forces, is
approximately 4000 years old. Ewing, The Fountain Pen: A
Collector's Companion (Running Press Book Publishers, Philadelphia,
1997). It has been used extensively throughout history to transport
molecules on macroscale dimensions.
[0006] By the present invention, these two related but, with regard
to scale and transport mechanism, disparate concepts have been
merged to create "dip pen" nanolithography (DPN). DPN utilizes a
scanning probe microscope (SPM) tip (e.g., an atomic force
microscope (AFM) tip) as a "nib" or "pen," a solid-state substrate
(e.g., gold) as "paper," and molecules with a chemical affinity for
the solid-state substrate as "ink." Capillary transport of
molecules from the tip to the solid substrate is used in DPN to
directly write patterns consisting of a relatively small collection
of molecules in submicrometer dimensions.
[0007] DPN is not the only lithographic method that allows one to
directly transport molecules to substrates of interest in a
positive printing mode. For example, microcontact printing, which
uses an elastomer stamp, can deposit patterns of
thiol-functionalized molecules directly onto gold substrates. Xia
et al., Angew. Chem. Int. Ed. Engl., 37:550 (1998); Kim et al.,
Nature, 376:581 (1995); Xia et al., Science, 273:347 (1996); Yan et
al., J. Am. Chem. Soc., 120:6179 (1998); Kumar et al., J. Am. Chem.
Soc., 114:9188 (1992). This method is a parallel technique to DPN,
allowing one to deposit an entire pattern or series of patterns on
a substrate of interest in one step. This is an advantage over a
serial technique like DPN, unless one is trying to selectively
place different types of molecules at specific sites within a
particular type of nanostructure. In this regard, DPN complements
microcontact printing and many other existing methods of micro- and
nanofabrication.
[0008] There are also a variety of negative printing techniques
that rely on scanning probe instruments, electron beams, or
molecular beams to pattern substrates using self-assembling
monolayers and other organic materials as resist layers (i.e., to
remove material for subsequent processing or adsorption steps).
Bottomley, Anal. Chem., 70:425R (1998); Nyffenegger et al., Chem.
Rev., 97:1195 (1997); Berggren et al., Science, 269:1255 (1995);
Sondag-Huethorst et al., Appl. Phys. Lett., 64:285 (1994); Schoer
et al., Langmuir, 13:2323 (1997); Xu et al., Langmuir, 13:127
(1997); Perkins et al., Appl. Phys. Lett., 68:550 (1996); Carr et
al., J. Vac. Sci. Technol. A, 15:1446 (1997); Lercel et al., Appl.
Phys. Lett., 68:1504 (1996); Sugimura et al., J. Vac. Sci. Technol.
A, 14:1223 (1996); Komeda et al., J. Vac. Sci. Technol. A, 16:1680
(1998); Muller et al., J. Vac. Sci. Technol. B, 13:2846 (1995); Kim
et al., Science, 257:375 (1992). However, DPN can deliver
relatively small amounts of a molecular substance to a substrate in
a nanolithographic fashion that does not rely on a resist, a stamp,
complicated processing methods, or sophisticated noncommercial
instrumentation.
[0009] A problem that has plagued AFM since its invention is the
narrow gap capillary formed between an AFM tip and sample when an
experiment is conducted in air which condenses water from the
ambient and significantly influences imaging experiments,
especially those attempting to achieve nanometer or even angstrom
resolution. Xu et al., J. Phys. Chem. B, 102:540 (1998); Binggeli
et al., Appl. Phys. Lett, 65:415 (1994); Fujihira et al., Chem.
Lett., 499 (1996); Piner et al., Langmuir, 13:6864 (1997). It has
been shown that this is a dynamic problem, and water, depending
upon relative humidity and substrate wetting properties, will
either be transported from the substrate to the tip or vice versa.
In the latter case, metastable, nanometer-length-scale patterns,
could be formed from very thin layers of water deposited from the
AFM tip (Finer et al., Langmuir, 13:6864 (1997)). The present
invention shows that, when the transported molecules can anchor
themselves to the substrate, stable surface structures are formed,
resulting in a new type of nanolithography, DPN.
[0010] The present invention also overcomes the problems caused by
the water condensation that occurs when performing AFM. In
particular, it has been found that the resolution of AFM is
improved considerably when the AFM tip is coated with certain
hydrophobic compounds prior to performing AFM.
SUMMARY OF THE INVENTION
[0011] As noted above, the invention provides a method of
lithography referred to as "dip pen" nanolithography, or DPN. DPN
is a direct-write, nanolithography technique by which molecules are
delivered to a substrate of interest in a positive printing mode.
DPN utilizes a solid substrate as the "paper" and a scanning probe
microscope (SPM) tip (e.g., an atomic force microscope (AFM) tip)
as the "pen". The tip is coated with a patterning compound (the
"ink"), and the coated tip is contacted with the substrate so that
the patterning compound is applied to the substrate to produce a
desired pattern. The molecules of the patterning compound are
delivered from the tip to the substrate by capillary transport. DPN
is useful in the fabrication of a variety of microscale and
nanoscale devices. The invention also provides substrates patterned
by DPN and kits for performing DPN.
[0012] The invention further provides a method of performing AFM
imaging in air. The method comprises coating an AFM tip with a
hydrophobic compound. Then, AFM imaging is performed in air using
the coated tip. The hydrophobic compound is selected so that AFM
imaging performed using the coated AFM tip is improved compared to
AFM imaging performed using an uncoated AFM tip. Finally, the
invention provides AFM tips coated with the hydrophobic
compounds.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1. Schematic representation of "dip pen"
nanolithography (DPN). A water meniscus forms between the atomic
force microscope (AFM) tip coated with 1-octadecanethiol (ODT) and
the gold (Au) substrate. The size of the meniscus, which is
controlled by relative humidity, affects the ODT transport rate,
the effective tip substrate contact area, and DPN resolution.
[0014] FIG. 2A. Lateral force image of a 1 .mu.m by 1 .mu.m square
of ODT deposited onto a Au substrate by DPN. This pattern was
generated by scanning the 1 .mu.m.sup.2 area at a scan rate of 1 Hz
for a period of 10 min at a relative humidity of 39%. Then the scan
size was increased to 3 .mu.m, and the scan rate was increased to 4
Hz while recording the image. The faster scan rate prevents ODT
transport.
[0015] FIG. 2B. Lattice resolved, lateral force image of an ODT
self-assembling monolayer (SAM) deposited onto a Au(111)/mica
substrate by DPN. The image has been filtered with a fast fourier
transform (FFT), and the FFT of the raw data is shown in the lower
right insert. The monolayer was generated by scanning a 1000 .ANG.
square area of the Au(111)/mica substrate five times at a rate of 9
Hz under 39% relative humidity.
[0016] FIG. 2C. Lateral force image of 30 nm wide line (3 .mu.m
long) deposited onto a Au/mica substrate by DPN. The line was
generated by scanning the tip in a vertical line repeatedly for
five minutes at a scan rate of 1 Hz.
[0017] FIG. 2D. Lateral force image of a 100 nm line deposited on a
Au substrate by DPN. The method of depositing this line is
analogous to that used to generate the image in FIG. 2C, but the
writing time was 1.5 minutes. Note that in all images (FIGS.
2A-2D), darker regions correspond to areas of relatively lower
friction.
[0018] FIG. 3A. Lateral force image of a Au substrate after an AFM
tip, which has been coated with ODT, has been in contact with the
substrate for 2, 4, and 16 min (left to right). The relative
humidity was held constant at 45%, and the image was recorded at a
scan rate of 4 Hz.
[0019] FIG. 3B. Lateral force image of 16-mercaptohexadecanoic acid
(MHDA) dots on a Au substrate. To generate the dots, a MHDA-coated
AFM tip was held on the Au substrate for 10, 20, and 40 seconds
(left to right). The relative humidity was 35%. Note that the
transport properties of MHDA and ODT differ substantially.
[0020] FIG. 3C. Lateral force image of an array of dots generated
by DPN. Each dot was generated by holding an ODT-coated tip in
contact with the surface for -20 seconds. Writing and recording
conditions were the same as in FIG. 3A.
[0021] FIG. 3D. Lateral force image of a molecule-based grid. Each
line, 100 nm in width and 2 .mu.m in length, required 1.5 minutes
to write.
[0022] FIGS. 4A-B. Oscilloscope recordings of lateral force
detector output before the AFM tip was coated with 1-dodecylamine
(FIG. 4A) and after the tip had been coated with 1-dodecylamine
(FIG. 4B). The time of the recording spans four scan lines. Since
the signal was recorded during both left and right scans, the
heights of the square waves are directly proportional to the
friction. The Y-axis zero has been shifted for clarity.
[0023] FIGS. 5A-B. Lateral force images showing water transported
to a glass substrate (dark area) by an unmodified AFM tip (FIG. 5A)
and the result of the same experiment performed with a
1-dodecylamine-coated tip (FIG. 5B). Height bars are in arbitrary
units.
[0024] FIG. 6A. Lattice resolved, lateral force image of a mica
surface with a 1-dodecylamine-coated tip. The 2D fourier transform
is in the insert.
[0025] FIG. 6B. Lattice resolved, lateral force image of an
self-assembled monolayer of 11-mercapto-1-undecanol. This image has
been fourier transform filtered (FFT), and the FFT of the raw data
is shown in lower right insert. Scale bars are arbitrary.
[0026] FIG. 6C. Topographic image of water condensation on mica at
30% relative humidity. The height bar is 5 .ANG..
[0027] FIG. 6D. Lateral force image of water condensation on mica
at 30% relative humidity (same spot as in FIG. 6C).
[0028] FIG. 7A-B. Topographic images of latex spheres, showing no
changes before and after modifying tip with 1-dodecylamine. Height
bars are 0.1 .mu.m. FIG. 7A was recorded with a clean tip, and FIG.
7B was recorded with the same tip coated with 1-dodecylamine.
[0029] FIGS. 8A-B. Images of a Si.sub.3N.sub.4 surface coated with
1-dodecylamine molecules, showing uniform coating. FIG. 8A shows
the topography of a Si.sub.3N.sub.4 wafer surface that has been
coated with the 1-dodecylamine molecules, which has similar
features as before coating. Height bar is 700 .ANG.. FIG. 8B shows
the same area recorded in lateral force mode, showing no
distinctive friction variation.
[0030] FIGS. 9A-C. Schematic diagrams with lateral force microscopy
(LFM) images of nanoscale molecular dots showing the "essential
factors" for nanometer scale multiple patterning by DPN. Scale bar
is 100 nm. FIG. 9A shows a first pattern of 15 nm diameter
1,16-mercaptohexadecanoic acid (MHA) dots on Au(111) imaged by LFM
with the MHA-coated tip used to make the dots. FIG. 9B shows a
second pattern written by DPN using a coordinate for the second
pattern calculated based on the LFM image of the first pattern
shown in FIG. 9A. FIG. 9C shows the final pattern comprising both
the first and second patterns. The elapsed time between forming the
two patterns was 10 minutes.
[0031] FIGS. 10A-C. For these figures, scale bar is 100 nm. FIG.
10A shows a first pattern comprised of 50 nm width lines and
alignment marks generated with MHA molecules by DPN. FIG. 10B shows
a second pattern generated with ODT molecules. The coordinates of
the second pattern were adjusted based on the LFM image of the MHA
alignment pattern. The first line patterns were not imaged to
prevent the possible contamination by the second molecules. FIG.
10C shows the final results comprising interdigitated 50 nm width
lines separated by 70 nm.
[0032] FIG. 11A. Letters drawn by DPN with MHA molecules on
amorphous gold surface. Scale bar is 100 nm, and the line width is
15 nm.
[0033] FIG. 11B. Polygons drawn by DPN with MHA molecules on
amorphous gold surface. ODT molecules were overwritten around the
polygons. Scale bar is 1 .mu.m, and the line width is 100 nm.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
[0034] DPN utilizes a scanning probe microscope (SPM) tip. As used
herein, the phrases "scanning probe microscope tip" and "SPM tip"
refer to tips used in atomic scale imaging, including atomic force
microscope (AFM) tips, near field scanning optical microscope
(NSOM) tips, scanning tunneling microscope (STM) tips, and devices
having similar properties. Many SPM tips are available
commercially, and similar devices may be developed using the
guidelines provided herein.
[0035] Most preferably, the SPM tip is an AFM tip. Any AFM tip can
be used. Suitable AFM tips include those that are available
commercially from, e.g., Park Scientific, Digital Instruments and
Molecular Imaging.
[0036] Also preferred are NSOM tips. These tips are hollow, and the
patterning compounds accumulate in the hollows of the NSOM tips
which serve as reservoirs of the patterning compound to produce a
type of "fountain pen" for use in DPN. Suitable NSOM tips are
available from Nanonics Ltd. and Topometrix.
[0037] The tip preferably is one to which the patterning compound
physisorbs only. As used herein "physisorb" means that the
patterning compound adheres to the tip surface by a means other
than as a result of a chemical reaction (i.e., no chemisorption or
covalent linkage) and can be removed from the tip surface with a
suitable solvent. Physisorption of the patterning compounds to the
tip can be enhanced by coating the tip with an adhesion layer and
by proper choice of solvent (when one is used) for the patterning
compound. The adhesion layer is a uniform, thin (<10 nm) layer
of material deposited on the tip surface which does not
significantly change the tip's shape. It should also be strong
enough to tolerate AFM operation (force of about 10 nN). Titanium
and chromium form very thin uniform layers on tips without changing
tip shape significantly, and are well-suited to be used to form the
adhesion layer. The tips can be coated with an adhesion layer by
vacuum deposition (see Holland, Vacuum Deposition Of Thin Films
(Wiley, New York, N.Y., 1956)), or any other method of forming thin
metal films. By "proper solvent" is meant a solvent that adheres to
(wets) the tip well. The proper solvent will vary depending on the
patterning compound used, the type of tip used, whether or not the
tip is coated with an adhesion layer, and the material used to form
the adhesion layer. For example, acetonitrile adheres well to
uncoated silicon nitride tips, making the use of an adhesion layer
unnecessary when acetonitrile is used as the solvent for a
patterning compound. In contrast, water does not adhere to uncoated
silicon nitride tips. Water does adhere well to titanium-coated
silicon nitride tips, and such coated tips can be used when water
is used as the solvent. Physisorption of aqueous solutions of
patterning compounds can also be enhanced by increasing the
hydrophilicity of the tips (whether coated or uncoated with an
adhesion layer). For instance, hydrophilicity can be increased by
cleaning the tips (e.g., with a piranha solution, by plasma
cleaning, or with UV ozone cleaning) or by oxygen plasma etching.
See Lo et al., Langmuir, 15, 6522-6526 (1999); James et al.,
Langmuir, 14, 741-744 (1998). Alternatively, a mixture of water and
another solvent (e.g., 1:3 ratio of water:acetonitrile) may adhere
to uncoated silicon nitride tips, making the use of an adhesion
layer or treatment to increase hydrophilicity unnecessary. The
proper solvent for a particular set of circumstances can be
determined empirically using the guidance provided herein.
[0038] The substrate may be of any shape and size. In particular,
the substrate may be flat or curved. Substrates may be made of any
material which can be modified by a patterning compound to form
stable surface structures (see below). Substrates useful in the
practice of the invention include metals (e.g., gold, silver,
aluminum, copper, platinum, and paladium), metal oxides (e.g.,
oxides of Al, Ti, Fe, Ag, Zn, Zr, In, Sn and Cu), semiconductor
materials (e.g., Si, CdSe, CdS and CdS coated with ZnS), magnetic
materials (e.g., ferromagnetite), polymers or polymer-coated
substrates, superconductor materials
(YBa.sub.2Cu.sub.3O.sub.7-.delta.), Si, SiO.sub.2, glass, AgI,
AgBr, HgI.sub.2, PbS, PbSe, ZnSe, ZnS, ZnTe, CdTe, InP,
In.sub.2O.sub.3/SnO.sub.2, In.sub.2S.sub.3, In.sub.2Se.sub.3,
In.sub.2Te.sub.3, Cd.sub.3P.sub.2, Cd.sub.3As.sub.2, InAs, AlAs,
GaP, and GaAs. Methods of making such substrates are well-known in
the art and include evaporation and sputtering (metal films),
crystal semiconductor growth (e.g., Si, Ge, GaAs), chemical vapor
deposition (semiconductor thin films), epitaxial growth
(crystalline semiconductor thin films), and thermal shrinkage
(oriented polymers). See, e.g., Alcock et al., Canadian
Metallurgical Quarterly, 23, 309 (1984); Holland, Vacuum Deposition
of Thin Films (Wiley, New York 1956); Grove, Philos. Trans. Faraday
Soc., 87 (1852); Teal, IEEE Trans. Electron Dev. ED-23, 621 (1976);
Sell, Key Eng. Materials, 58, 169 (1991); Keller et al., Float-Zone
Silicon (Marcel Dekker, New York, 1981); Sherman, Chemical Vapor
Deposition For Microelectronics: Principles, Technology And
Applications (Noyes, Park Ridges, N.J., 1987); Epitaxial Silicon
Technology (Baliga, ed., Academic Press, Orlando, Fla., 1986); U.S.
Pat. No. 5,138,174; Hidber et al., Langmuir, 12, 5209-5215 (1996).
Suitable substrates can also be obtained commercially from, e.g.,
Digital Instruments (gold), Molecular Imaging (gold), Park
Scientific (gold), Electronic Materials, Inc. (semiconductor
wafers), Silicon Quest, Inc. (semiconductor wafers), MEMS
Technology Applications Center, Inc. (semiconductor wafers),
Crystal Specialties, Inc. (semiconductor wafers), Siltronix,
Switzerland (silicon wafers), Aleene's, Buellton, Calif.
(biaxially-oriented polystyrene sheets), and Kama Corp., Hazelton,
Pa. (oriented thin films of polystyrene).
[0039] The SPM tip is used to deliver a patterning compound to a
substrate of interest. Any patterning compound can be used,
provided it is capable of modifying the substrate to form stable
surface structures. Stable surface structures are formed by
chemisorption of the molecules of the patterning compound onto the
substrate or by covalent linkage of the molecules of the patterning
compound to the substrate.
[0040] Many suitable compounds which can be used as the patterning
compound, and the corresponding substrate(s) for the compounds, are
known in the art. For example: [0041] a. Compounds of the formula
R.sub.1SH, R.sub.1SSR.sub.2, R.sub.1SR.sub.2, R.sub.1SO.sub.2H,
(R.sub.1).sub.3P, R.sub.1NC, R.sub.1CN, (R.sub.1).sub.3N,
R.sub.1COOH, or ArSH can be used to pattern gold substrates; [0042]
b. Compounds of formula R.sub.1SH, (R.sub.1).sub.3N, or ArSH can be
used to pattern silver, copper, palladium and semiconductor
substrates; [0043] c. Compounds of the formula R.sub.1NC,
R.sub.1SH, R.sub.1SSR.sub.2, or R.sub.1SR.sub.2 can be used to
pattern platinum substrates; [0044] d. Compounds of the formula
R.sub.1SH can be used to pattern aluminum, TiO.sub.2, SiO.sub.2,
GaAs and InP substrates; [0045] e. Organosilanes, including
compounds of the formula R.sub.1SiCl.sub.3,
R.sub.1Si(OR.sub.2).sub.3, (R.sub.1COO).sub.2,
R.sub.1CH.dbd.CH.sub.2, R.sub.1Li or R.sub.1MgX, can be used to
pattern Si, SiO.sub.2 and glass substrates; [0046] f. Compounds of
the formula R.sub.1COOH or R.sub.1CONHR.sub.2 can be used to
pattern metal oxide substrates; [0047] g. Compounds of the formula
R.sub.1SH, R.sub.1NH.sub.2, ArNH.sub.2, pyrrole, or pyrrole
derivatives wherein R.sub.1 is attached to one of the carbons of
the pyrrole ring, can be used to pattern cuprate high temperature
superconductors; [0048] h. Compounds of the formula
R.sub.1PO.sub.3H.sub.2 can be used to pattern ZrO.sub.2 and
In.sub.2O.sub.3/SnO.sub.2 substrates; [0049] i. Compounds of the
formula R.sub.1COOH can be used to pattern aluminum, copper,
silicon and platinum substrates; [0050] j. Compounds that are
unsaturated, such as azoalkanes (R.sub.3NNR.sub.3) and
isothiocyanates (R.sub.3NCS), can be used to pattern silicon
substrates; [0051] k. Proteins and peptides can be used to pattern,
gold, silver, glass, silicon, and polystyrene. In the above
formulas:
[0052] R.sub.1 and R.sub.2 each has the formula X(CH.sub.2)n and,
if a compound is substituted with both R.sub.1 and R.sub.2, then
R.sub.1 and R.sub.2 can be the same or different;
[0053] R.sub.3 has the formula CH.sub.3(CH.sub.2)n;
[0054] n is 0-30;
[0055] Ar is an aryl;
[0056] X is --CH.sub.3, --CHCH.sub.3, --COOH,
--CO.sub.2(CH.sub.2).sub.mCH.sub.3, --OH, --CH.sub.2OH, ethylene
glycol, hexa(ethylene glycol), --O(CH.sub.2).sub.mCH.sub.3,
--NH.sub.2, --NH(CH.sub.2).sub.mNH.sub.2, halogen, glucose,
maltose, fullerene C60, a nucleic acid (oligonucleotide, DNA, RNA,
etc.), a protein (e.g., an antibody or enzyme) or a ligand (e.g.,
an antigen, enzyme substrate or receptor); and
[0057] m is 0-30.
[0058] For a description of patterning compounds and their
preparation and use, see Xia and Whitesides, Angew. Chem. Int. Ed.,
37, 550-575 (1998) and references cited therein; Bishop et al.,
Curr. Opinion Colloid & Interface Sci., 1, 127-136 (1996);
Calvert, J. Vac. Sci. Technol. B, 11, 2155-2163 (1993); Ulman,
Chem. Rev., 96:1533 (1996) (alkanethiols on gold); Dubois et al.,
Annu. Rev. Phys. Chem., 43:437 (1992) (alkanethiols on gold);
Ulman, An Introduction to Ultrathin Organic Films: From
Langmuir-Blodgett to Self-Assembly (Academic, Boston, 1991)
(alkanethiols on gold); Whitesides, Proceedings of the Robert A.
Welch Foundation 39th Conference On Chemical Research Nanophase
Chemistry, Houston, Tex., pages 109-121 (1995) (alkanethiols
attached to gold); Mucic et al. Chem. Commun. 555-557 (1996)
(describes a method of attaching 3' thiol DNA to gold surfaces);
U.S. Pat. No. 5,472,881 (binding of
oligonucleotide-phosphorothiolates to gold surfaces); Burwell,
Chemical Technology, 4, 370-377 (1974) and Matteucci and Caruthers,
J. Am. Chem. Soc., 103, 3185-3191 (1981) (binding of
oligonucleotides-alkylsiloxanes to silica and glass surfaces);
Grabar et al., Anal. Chem., 67, 735-743 (binding of
aminoalkylsiloxanes and for similar binding of
mercaptoalkylsiloxanes); Nuzzo et al., J. Am. Chem. Soc., 109, 2358
(1987) (disulfides on gold); Allara and Nuzzo, Langmuir, 1, 45
(1985) (carboxylic acids on aluminum); Allara and Tompkins, J.
Colloid Interface Sci., 49, 410-421 (1974) (carboxylic acids on
copper); Iler, The Chemisty Of Silica, Chapter 6, (Wiley 1979)
(carboxylic acids on silica); Timmons and Zisman, J. Phys. Chem.,
69, 984-990 (1965) (carboxylic acids on platinum); Soriaga and
Hubbard, J. Am. Chem. Soc., 104, 3937 (1982) (aromatic ring
compounds on platinum); Hubbard, Acc. Chem. Res., 13, 177 (1980)
(sulfolanes, sulfoxides and other functionalized solvents on
platinum); Hickman et al., J. Am. Chem. Soc., 111, 7271 (1989)
(isonitriles on platinum); Maoz and Sagiv, Langmuir, 3, 1045 (1987)
(silanes on silica); Maoz and Sagiv, Langmuir, 3, 1034 (1987)
(silanes on silica); Wasserman et al., Langmuir, 5, 1074 (1989)
(silanes on silica); Eltekova and Eltekov, Langmuir, 3, 951 (1987)
(aromatic carboxylic acids, aldehydes, alcohols and methoxy groups
on titanium dioxide and silica); and Lec et al., J. Phys. Chem.,
92, 2597 (1988) (rigid phosphates on metals); Lo et al., J. Am.
Chem. Soc., 118, 11295-11296 (1996) (attachment of pyrroles to
superconductors); Chen et al., J. Am. Chem. Soc., 117, 6374-5
(1995) (attachment of amines and thiols to superconductors); Chen
et al., Langmuir, 12, 2622-2624 (1996) (attachment of thiols to
superconductors); McDevitt et al., U.S. Pat. No. 5,846,909
(attachment of amines and thiols to superconductors); Xu et al.,
Langmuir, 14, 6505-6511 (1998) (attachment of amines to
superconductors); Mirkin et al., Adv. Mater. (Weinheim, Ger.), 9,
167-173 (1997) (attachment of amines to superconductors); Hovis et
al., J. Phys. Chem. B, 102, 6873-6879 (1998) (attachment of olefins
and dienes to silicon); Hovis et al., Surf Sci., 402-404, 1-7
(1998) (attachment of olefins and dienes to silicon); Hovis et al.,
J. Phys. Chem. B, 101, 9581-9585 (1997) (attachment of olefins and
dienes to silicon); Hamers et al., J. Phys. Chem. B, 101, 1489-1492
(1997) (attachment of olefins and dienes to silicon); Hamers et
al., U.S. Pat. No. 5,908,692 (attachment of olefins and dienes to
silicon); Ellison et al., J. Phys. Chem. B, 103, 6243-6251 (1999)
(attachment of isothiocyanates to silicon); Ellison et al., J.
Phys. Chem. B, 102, 8510-8518 (1998) (attachment of azoalkanes to
silicon); Ohno et al., Mol. Cryst. Liq. Cryst. Sci. Technol., Sect.
A, 295, 487-490 (1997) (attachment of thiols to GaAs); Reuter et
al., Mater. Res. Soc. Symp. Proc., 380, 119-24 (1995) (attachment
of thiols to GaAs); Bain, Adv. Mater. (Weinheim, Fed. Repub. Ger.),
4, 591-4 (1992) (attachment of thiols to GaAs); Sheen et al., J.
Am. Chem. Soc., 114, 1514-15 (1992) (attachment of thiols to GaAs);
Nakagawa et al., Jpn. J. Appl. Phys., Part 1, 30, 3759-62 (1991)
(attachment of thiols to GaAs); Lunt et al., J. Appl. Phys., 70,
7449-67 (1991) (attachment of thiols to GaAs); Lunt et al., J. Vac.
Sci. Technol., B, 9, 2333-6 (1991) (attachment of thiols to GaAs);
Yamamoto et al., Langmuir ACS ASAP, web release number Ia990467r
(attachment of thiols to InP); Gu et al., J. Phys. Chem. B, 102,
9015-9028 (1998) (attachment of thiols to InP); Menzel et al., Adv.
Mater. (Weinheim, Ger.), 11, 131-134 (1999) (attachment of
disulfides to gold); Yonezawa et al., Chem. Mater., 11, 33-35
(1999) (attachment of disulfides to gold); Porter et al., Langmuir,
14, 7378-7386 (1998) (attachment of disulfides to gold); Son et
al., J. Phys. Chem., 98, 8488-93 (1994) (attachment of nitriles to
gold and silver); Steiner et al., Langmuir, 8, 2771-7 (1992)
(attachment of nitrites to gold and copper); Solomun et al., J.
Phys. Chem., 95, 10041-9 (1991) (attachment of nitriles to gold);
Solomun et al., Ber. Bunsen-Ges. Phys. Chem., 95, 95-8 (1991)
(attachment of nitriles to gold); Henderson et al., Inorg. Chim.
Acta, 242, 115-24 (1996) (attachment of isonitriles to gold); Huc
et al., J. Phys. Chem. B, 103, 10489-10495 (1999) (attachment of
isonitriles to gold); Hickman et al., Langmuir, 8, 357-9 (1992)
(attachment of isonitriles to platinum); Steiner et al., Langmuir,
8, 90-4 (1992) (attachment of amines and phospines to gold and
attachment of amines to copper); Mayya et al., J. Phys. Chem. B,
101, 9790-9793 (1997) (attachment of amines to gold and silver);
Chen et al., Langmuir, 15, 1075-1082 (1999) (attachment of
carboxylates to gold); Tao, J. Am. Chem. Soc., 115, 4350-4358
(1993) (attachment of carboxylates to copper and silver); Laibinis
et al., J. Am. Chem. Soc., 114, 1990-5 (1992) (attachment of thiols
to silver and copper); Laibinis et al., Langmuir, 7, 3167-73 (1991)
(attachment of thiols to silver); Fenter et al., Langmuir, 7,
2013-16 (1991) (attachment of thiols to silver); Chang et al., Am.
Chem. Soc., 116, 6792-805 (1994) (attachment of thiols to silver);
Li et al., J. Phys. Chem., 98, 11751-5 (1994) (attachment of thiols
to silver); Li et al., Report, 24 pp (1994) (attachment of thiols
to silver); Tarlov et al., U.S. Pat. No. 5,942,397 (attachment of
thiols to silver and copper); Waldeck, et al., PCT application
WO/99/48682 (attachment of thiols to silver and copper); Gui et
al., Langmuir, 7, 955-63 (1991) (attachment of thiols to silver);
Walczak et al., J. Am. Chem. Soc., 113, 2370-8 (1991) (attachment
of thiols to silver); Sangiorgi et al., Gazz. Chim. Ital., 111,
99-102 (1981) (attachment of amines to copper); Magallon et al.,
Book of Abstracts, 215th ACS National Meeting, Dallas, Mar. 29-Apr.
2, 1998, COLL-048 (attachment of amines to copper); Patil et al.,
Langmuir, 14, 2707-2711 (1998) (attachment of amines to silver);
Sastry et al., J. Phys. Chem. B, 101, 4954-4958 (1997) (attachment
of amines to silver); Barisal et al., J. Phys. Chem. B, 102,
4058-4060 (1998) (attachment of alkyl lithium to silicon); Bansal
et al., J. Phys. Chem. B, 102, 1067-1070 (1998) (attachment of
alkyl lithium to silicon); Chidsey, Book of Abstracts, 214th ACS
National Meeting, Las Vegas, Nev., Sep. 7-11, 1997, I&EC-027
(attachment of alkyl lithium to silicon); Song, J. H., Thesis,
University of California at San Diego (1998) (attachment of alkyl
lithium to silicon dioxide); Meyer et al., J. Am. Chem. Soc., 110,
4914-18 (1988) (attachment of amines to semiconductors); Brazdil et
al. J. Phys. Chem., 85, 1005-14 (1981) (attachment of amines to
semiconductors); James et al., Langmuir, 14, 741-744 (1998)
(attachment of proteins and peptides to glass); Bernard et al.,
Langmuir, 14, 2225-2229 (1998) (attachment of proteins to glass,
polystyrene, gold, silver and silicon wafers).
[0059] Other compounds known in the art besides those listed above,
or which are developed or discovered using the guidelines provided
herein or otherwise, can also be used as the patterning compound.
Presently preferred are alkanethiols and aryithiols on a variety of
substrates and trichlorosilanes on SiO.sub.2 substrates (see
Examples 1 and 2).
[0060] To practice DPN, the SPM tip is coated with a patterning
compound. This can be accomplished in a number of ways. For
instance, the tip can be coated by vapor deposition, by direct
contact scanning, or by bringing the tip into contact with a
solution of the patterning compound.
[0061] The simplest method of coating the tips is by direct contact
scanning. Coating by direct contact scanning is accomplished by
depositing a drop of a saturated solution of the patterning
compound on a solid substrate (e.g., glass or silicon nitride;
available from Fisher Scientific or MEMS Technology Application
Center). Upon drying, the patterning compound forms a
microcrystalline phase on the substrate. To coat the patterning
compound on the SPM tip, the tip is scanned repeatedly across this
microcrystalline phase. While this method is simple, it does not
lead to the best loading of the tip, since it is difficult to
control the amount of patterning compound transferred from the
substrate to the tip.
[0062] The tips can also be coated by vapor deposition. See
Sherman, Chemical Vapor Deposition For Microelectronics:
Principles, Technology And Applications (Noyes, Park Ridges, N.J.,
1987. Briefly, a patterning compound (in pure form, solid or
liquid, no solvent) is placed on a solid substrate (e.g., glass or
silicon nitride; obtained from Fisher Scientific or MEMS Technology
Application Center), and the tip is position near (within about
1-20 cm, depending on chamber design) the patterning compound. The
compound is then heated to a temperature at which it vaporizes,
thereby coating the tip with the compound. For instance,
1-octadecanethiol can be vapor deposited at 60.degree. C. Coating
by vapor deposition should be performed in a closed chamber to
prevent contamination of other areas. If the patterning compound is
one which is oxidized by air, the chamber should be a vacuum
chamber or a nitrogen-filled chamber. Coating the tips by vapor
deposition produces thin, uniform layers of patterning compounds on
the tips and gives very reliable results in DPN.
[0063] Preferably, however, the SPM tip is coated by dipping the
tip into a solution of the patterning compound. The solvent is not
critical; all that is required is that the compound be in solution.
However, the solvent is preferably the one in which the patterning
compound is most soluble. Also, the solution is preferably a
saturated solution. In addition, the solvent is preferably one that
adheres to (wets) the tip (uncoated or coated with an adhesion
layer) very well (see above). The tip is maintained in contact with
the solution of the patterning compound for a time sufficient for
the compound to coat the tip. Such times can be determined
empirically. Generally, from about 30 seconds to about 3 minutes is
sufficient. Preferably, the tip is dipped in the solution multiple
times, with the tip being dried between each dipping. The number of
times a tip needs to be dipped in a chosen solution can be
determined empirically. Preferably, the tip is dried by blowing an
inert gas (such as carbon tetrafluoride,
1,2-dichloro-1,1,2,2,-tetrafluoroethane, dichlorodifluoromethane,
octafluorocyclobutane, trichlorofluoromethane, difluoroethane,
nitrogen, nitrogen, argon or dehumidified air) not containing any
particles (i.e., purified) over the tip. Generally, about 10
seconds of blowing with the gas at room temperature is sufficient
to dry the tip. After dipping (the single dipping or the last of
multiple dippings), the tip may be used wet to pattern the
substrate, or it may be dried (preferably as described above)
before use. A dry tip gives a low, but stable, rate of transport of
the patterning compound for a long time (on the order of weeks),
whereas a wet tip gives a high rate of transport of the patterning
compound for a short time (about 2-3 hours). A dry tip is preferred
for compounds having a good rate of transport under dry conditions
(such as the compounds listed above wherein X=--CH.sub.3), whereas
a wet tip is preferred for compounds having a low rate of transport
under dry conditions (such as the compounds listed above wherein
X=--COOH).
[0064] To perform DPN, the coated tip is brought into contact with
a substrate. Both the patterning compound and a transport medium
are necessary for DPN since the patterning compound is transported
to the substrate by capillary transport (see FIG. 1). The transport
medium forms a meniscus which bridges the gap between the tip and
the substrate (see FIG. 1). Thus, the tip is "in contact" with the
substrate when it is close enough so that this meniscus forms.
Suitable transport media include water, hydrocarbons (e.g.,
hexane), and solvents in which the patterning compounds are soluble
(e.g., the solvent used for coating the tip--see above). Faster
writing with the tip can be accomplished by using the transport
medium in which the patterning compound is most soluble.
[0065] Single tips can be used to write a pattern utilizing an AFM
or similar device. As is known in the art, only some STM and NSOM
tips can be used in an AFM, and STM and NSOM tips which can be used
in an AFM are available commercially. Two or more different
patterning compounds can be applied to the same substrate to form
patterns (the same or different) of the different compounds by
removing the first tip coated with a first patterning compound and
replacing it with another tip coated with a different patterning
compound. Alternatively, a plurality of tips can be used in a
single device to write a plurality of patterns (the same pattern or
different patterns) on a substrate using the same or different
patterning compounds. See, e.g., U.S. Pat. No. 5,666,190, which
describes a device comprising multiple cantilevers and tips for
patterning a substrate.
[0066] When two or more patterns and/or two or more patterning
compounds (in the same or different patterns) are applied to a
single substrate, a positioning (registration) system is used to
align the patterns and/or patterning compounds relative to each
other and/or relative to selected alignment marks. For instance,
two or more alignment marks, which can be imaged by normal AFM
imaging methods, are applied to the substrate by DPN or another
lithographic technique (such as photolithography or e-beam
lithography). The alignment marks may be simple shapes, such as a
cross or rectangle. Better resolution is obtained by making the
alignment marks using DPN. If DPN is used, the alignment marks are
preferably made with patterning compounds which form strong
covalent bonds with the substrate. The best compound for forming
the alignment marks on gold substrates is 16-mercaptohexadecanoic
acid. The alignment marks are imaged by normal AFM methods (such as
lateral force AFM imaging, AFM topography imaging and non-contact
mode AFM imaging), preferably using an SPM tip coated with a
patterning compound for making a desired pattern. For this reason,
the patterning compounds used to make the alignment marks should
not react with the other patterning compounds which are to be used
to make the desired patterns and should not be destroyed by
subsequent DPN patterning. Using the imaging data, the proper
parameters (position and orientation) can be calculated using
simple computer programs (e.g., Microsoft Excel spreadsheet), and
the desired pattern(s) deposited on the substrate using the
calculated parameters. Virtually an infinite number of patterns
and/or patterning compounds can be positioned using the alignment
marks since the system is based on calculating positions and
orientations relative to the alignment marks. To get the best
results, the SPM tip positioning system which is used should be
stable and not have drift problems. One AFM positioning system
which meets these standards is the 100 micrometer pizoelectric tube
scanner available from Park Scientific. It provides stable
positioning with nanometer scale resolution.
[0067] DPN can also be used in a nanoplotter format by having a
series of micron-scale wells (or other containers) containing a
plurality of different patterning compounds and rinsing solutions
adjacent the substrate. The tip can be dipped into a well
containing a patterning compound to coat the tip, and the coated
tip is used to apply a pattern to the substrate. Then the tip is
rinsed by dipping it in a rinsing well or series of rinsing wells.
The rinsed tip is dipped into another well to be coated with a
second patterning compound and is then used to apply a pattern to
the substrate with the second patterning compound. The patterns are
aligned as described in the previous paragraph. The process of
coating the tip with a patterning compound, applying a pattern to
the substrate with this patterning compound, and rinsing the tip,
can be repeated as many times as desired, and the entire process
can be automated using appropriate software.
[0068] DPN can also be used to apply a second patterning compound
to a first patterning compound which has already been applied to a
substrate, whether by DPN or another method. The second patterning
compound is chosen so that it reacts chemically or otherwise stably
combines (e.g., by hybridization of two complimentary strands of
nucleic acid) with the first patterning compound. See, e.g., Dubois
and Nuzzo, Annu. Rev. Phys. Chem., 43, 437-63 (1992); Yan et al.,
Langmuir, 15, 1208-1214 (1999); Lahiri et al., Langmuir, 15,
2055-2060 (1999); and Huck et al., Langmuir, 15, 6862-6867 (1999).
As with DPN performed directly on a substrate, both the second
patterning compound and a transport medium are necessary, since the
second patterning compound is transported to the first patterning
compound by capillary transport (see above). Third, fourth, etc.,
patterning compounds can also be applied to the first patterning
compound, or to other patterning compounds, already on the
substrate. Further, additional patterning compounds can be applied
to form multiple layers of patterning compounds. Each of these
additional patterning compounds may be the same or different than
the other patterning compounds, and each of the multiple layers may
be the same or different than the other layers and may be composed
of one or more different patterning compounds.
[0069] Further, DPN can be used in combination with other
lithographic techniques. For instance, DPN can be used in
conjunction with microcontact printing and the other lithographic
techniques discussed in the Background section above.
[0070] Several parameters affect the resolution of DPN, and its
ultimate resolution is not yet clear. First, the grain size of the
substrate affects DPN resolution much like the texture of paper
controls the resolution of conventional writing. As shown in
Example 1 below, DPN has been used to make lines 30 nm in width on
a particular gold substrate. This size is the average grain
diameter of the gold substrate, and it represents the resolution
limit of DPN on this type of substrate. It is expected that better
resolution will be obtained using smoother (smaller grain size)
substrates, such as silicon. Indeed, using another, smoother gold
substrate, the resolution was increased to 15 nm (see Example
4).
[0071] Second, chemisorption, covalent attachment and self-assembly
all act to limit diffusion of the molecules after deposition. In
contrast, compounds, such as water, which do not anchor to the
substrate, form only metastable patterns of poor resolution (See
Piner et al., Langmuir, 13:6864 (1997)) and cannot be used.
[0072] Third, the tip-substrate contact time and, thus, scan speed
influence DPN resolution. Faster scan speeds and a smaller number
of traces give narrower lines.
[0073] Fourth, the rate of transport of the patterning compound
from the tip to the substrate affects resolution. For instance,
using water as the transport medium, it has been found that
relative humidity affects the resolution of the lithographic
process. For example, a 30-nm-wide line (FIG. 2C) required 5
minutes to generate in a 34% relative humidity environment, whereas
a 100-nm-line (FIG. 2D) required 1.5 minutes to generate in a 42%
relative humidity environment. It is known that the size of the
water meniscus that bridges the tip and substrate depends upon
relative humidity (Piper et al., Langmuir, 13:6864 (1997)), and the
size of the water meniscus affects the rate of transport of the
patterning compound to the substrate. Further, when a wet tip is
used, the water meniscus contains residual solvent is the transport
medium, and the transport rate is also affected by the properties
of the solvent.
[0074] Fifth, the sharpness of the tip also affects the resolution
of DPN. Thus, it is expected that better resolution will be
obtained using sharper tips (e.g., by changing the tips frequently,
cleaning the tips before coating them, and attaching sharp
structures (such as carbon nanotubes) to the ends of the tips).
[0075] In summary, DPN is a simple but powerful method for
transporting molecules from SPM tips to substrates at resolutions
comparable to those achieved with much more expensive and
sophisticated competitive lithographic methods, such as
electron-beam lithography. DPN is a useful tool for creating and
functionalizing microscale and nanoscale structures. For instance,
DPN can be used in the fabrication of microsensors, microreactors,
combinatorial arrays, micromechanical systems, microanalytical
systems, biosurfaces, biomaterials, microelectronics, microoptical
systems, and nanoelectronic devices. See, e.g., Xia and Whitesides,
Angew. Chem. Int. Ed., 37, 550-575 (1998). DPN should be especially
useful for the detailed functionalization of nanoscale devices
prepared by more conventional lithographic methods. See Reed et
al., Science, 278:252 (1997); Feldheim et al., Chem. Soc. Rev.,
27:1 (1998).
[0076] The invention also provides kits for performing DPN. The
kits comprise one or more substrates and one or more SPM tips. The
substrates and the tips are those described above. The tips may be
coated with a patterning compound or may be uncoated. If the tips
are uncoated, the kit may further comprise one or more containers,
each container holding a patterning compound. The patterning
compounds are those described above. Any suitable container can be
used, such as a vial, tube or jar. The kit may further comprise
materials for forming a thin solid adhesion layer to enhance
physisorption of the patterning compounds to the tips as described
above (such as a container of titanium or chromium), materials
useful for coating the tips with the patterning compounds (such as
solvents for the patterning compounds or solid substrates for
direct contact scanning), and/or materials for performing
lithography by methods other than DPN (see the Background section
and references cited therein). Finally, the kit may comprise other
reagents and items useful for performing DPN or any other
lithography method, such as reagents, beakers, vials, etc.
[0077] As noted above, when an AFM is operated in air, water
condenses between the tip and surface and then is transported by
means of the capillary as the tip is scanned across the surface.
This filled capillary, and the capillary force associated with it,
significantly impede the operation of the AFM and substantially
affect the imaging process.
[0078] Quite surprisingly, it has been found that AFM tips coated
with certain hydrophobic compounds exhibit an improved ability to
image substrates in air by AFM as compared to uncoated tips. The
reason for this is that the hydrophobic molecules reduce the size
of the water meniscus formed and effectively reduce friction. As a
consequence, the resolution of AFM in air is increased using a
coated tip, as compared to using an uncoated tip. Accordingly,
coating tips with the hydrophobic molecules can be utilized as a
general pretreatment for AFM tips for performing AFM in air.
[0079] Hydrophobic compounds useful for coating AFM tips for
performing AFM in air must form a uniform thin coating on the tip
surface, must not bind covalently to the substrate being imaged or
to the tip, must bind to the tip more strongly than to the
substrate, and must stay solid at the temperature of AFM operation.
Suitable hydrophobic compounds include those hydrophobic compounds
described above for use as patterning compounds, provided that such
hydrophobic patterning compounds are not used to coat AFM tips
which are used to image a corresponding substrate for the
patterning compound or to coat AFM tips which are made of, or
coated with, materials useful as the corresponding substrate for
the patterning compound. Preferred hydrophobic compounds for most
substrates are those having the formula R.sub.4NH.sub.2, wherein
R.sub.4 is an alkyl of the formula CH.sub.3(CH.sub.2).sub.n or an
aryl, and n is 0-30, preferably 10-20 (see discussion of patterning
compounds above). Particularly preferred is 1-dodecylamine for AFM
temperatures of operation below 74.degree. F. (about 23.3.degree.
C.).
[0080] AFM in air using any AFM tip may be improved by coating the
AFM tip with the hydrophobic compounds described in the previous
paragraph. Suitable AFM tips include those described above for use
in DPN.
[0081] AFM tips can be coated with the hydrophobic compounds in a
variety of ways. Suitable methods include those described above for
coating AFM tips with patterning compounds for use in DPN.
Preferably, the AFM tip is coated with a hydrophobic compound by
simply dipping the tip into a solution of the compound for a
sufficient time to coat the tip and then drying the coated tip with
an inert gas, all as described above for coating a tip with a
patterning compound.
[0082] After the tip is coated, AFM is performed in the same manner
as it would be if the tip were not coated. No changes in AFM
procedures have been found necessary.
EXAMPLES
Example 1
"Dip Pen" Nanolithography with Alkanethiols on a Gold Substrate
[0083] The transfer of 1-octadecanethiol (ODT) to gold (Au)
surfaces is a system that has been studied extensively. See Bain et
al., Angew. Chem. Int. Ed. Engl., 28:506 (1989); A. Ulman, An
Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to
Self-Assembly (Academic Press, Boston, 1991); Dubois et al., Annu.
Rev. Phys. Chem., 43:437 (1992); Bishop et al., Curr. Opin. Coll.
Interf. Sci., 1:127 (1996); Alves et al., J. Am. Chem. Soc.,
114:1222 (1992). Au having this moderately-air-stable molecule
immobilized on it can be easily differentiated from unmodified Au
by means of lateral force microscopy (LFM).
[0084] When an AFM tip coated with ODT is brought into contact with
a sample surface, the ODT flows from the tip to the sample by
capillary action, much like a dip pen (FIG. 1). This process has
been studied using a conventional AFM tip on thin film substrates
that were prepared by thermally evaporating 300 .ANG. of
polycrystalline Au onto mica at room temperature. A Park Scientific
Model CP AFM instrument was used to perform all experiments. The
scanner was enclosed in a glass isolation chamber, and the relative
humidity was measured with a hygrometer. All humidity measurements
have an absolute error of .+-.5%. A silicon nitride tip (Park
Scientific, Microlever A) was coated with ODT by dipping the
cantilever into a saturated solution of ODT in acetonitrile for 1
minute. The cantilever was blown dry with compressed difluoroethane
prior to use.
[0085] A simple demonstration of the DPN process involved raster
scanning a tip that was prepared in this manner across a 1 .mu.m by
1 .mu.m section of a Au substrate (FIG. 2A). An LFM image of this
section within a larger scan area (3 .mu.m by 3 .mu.m) showed two
areas of differing contrast (FIG. 2A). The interior dark area, or
region of lower lateral force, was a deposited monolayer of ODT,
and the exterior lighter area was bare Au.
[0086] Formation of high-quality self-assembled monolayers (SAMs)
occurred when the deposition process was carried out on
Au(111)/mica, which was prepared by annealing the Au thin film
substrates at 300.degree. C. for 3 hours. Alves et al., J. Am.
Chem. Soc., 114:1222 (1992). In this case, it was possible to
obtain a lattice-resolved image of an ODT SAM (FIG. 2B). The
hexagonal lattice parameter of 5.0.+-.0.2 .ANG. compares well with
reported values for SAMs of ODT on Au(111) (Id.) and shows that
ODT, rather than some other adsorbate (water or acetonitrile), was
transported from the tip to the substrate.
[0087] Although the experiments performed on Au(111)/mica provided
important information about the chemical identity of the
transported species in these experiments, Au(111)/mica is a poor
substrate for DPN. The deep valleys around the small Au(111) facets
make it difficult to draw long (micrometer) contiguous lines with
nanometer widths.
[0088] The nonannealed Au substrates are relatively rough
(root-mean square roughness=2 nm), but 30 nm lines could be
deposited by DPN (FIG. 2C). This distance is the average Au grain
diameter of the thin film substrates and represents the resolution
limit of DPN on this type of substrate. The 30-nm molecule-based
line prepared on this type of substrate was discontinuous and
followed the grain edges of the Au. Smoother and more contiguous
lines could be drawn by increasing the line width to 100 nm (FIG.
2D) or presumably by using a smoother Au substrate. The width of
the line depends upon tip scan speed and rate of transport of the
alkanethiol from the tip to the substrate (relative humidity can
change the transport rate). Faster scan speeds and a smaller number
of traces give narrower lines.
[0089] DPN was also used to prepare molecular dot features to
demonstrate the diffusion properties of the "ink" (FIGS. 3A and
3B). The ODT-coated tip was brought into contact (set point=1 nN)
with the Au substrate for a set period of time. For example, 0.66
.mu.m, 0.88 .mu.m, and 1.6 .mu.m diameter ODT dots were generated
by holding the tip in contact with the surface for 2, 4, and 16
minutes, respectively (left to right, FIG. 3A). The uniform
appearance of the dots likely reflects an even flow of ODT in all
directions from the tip to the surface. Opposite contrast images
were obtained by depositing dots of an alkanethiol derivative,
16-mercaptohexadecanoic acid in an analogous fashion (FIG. 3B).
This not only provides additional evidence that the molecules are
being transported from the tip to the surface but also demonstrates
the molecular generality of DPN.
[0090] Arrays and grids could be generated in addition to
individual lines and dots. An array of twenty-five 0.46-.mu.m
diameter ODT dots spaced 0.54 .mu.m apart (FIG. 3C) was generated
by holding an ODT-coated tip in contact with the surface (1 nM) for
20 seconds at 45% relative humidity without lateral movement to
form each dot. A grid consisting of eight intersecting lines 2
.mu.m in length and 100 nm wide (FIG. 3D) was generated by sweeping
the ODT-coated tip on a Au surface at a 4 .mu.m per second scan
speed with a 1 nN force for 1.5 minutes to form each line.
Example 2
"Dip Pen" Nanolithography
[0091] A large number of compounds and substrates have been
successfully utilized in DPN. They are listed below in Table 1,
along with possible uses for the combinations of compounds and
substrates.
[0092] AFM tips (Park Scientific) were used. The tips were silicon
tips, silicon nitride tips, and silicon nitride tips coated with a
10 nm layer of titanium to enhance physisorption of patterning
compounds. The silicon nitride tips were coated with the titanium
by vacuum deposition as described in Holland, Vacuum Deposition Of
Thin Films (Wiley, New York, N.Y., 1956). It should be noted that
coating the silicon nitride tips with titanium made the tips dull
and decreased the resolution of DPN. However, titanium-coated tips
are useful when water is used as the solvent for a patterning
compound. DPN performed with uncoated silicon nitride tips gave the
best resolution (as low as about 10 nm).
[0093] Metal film substrates listed in Table 1 were prepared by
vacuum deposition as described in Holland, Vacuum Deposition Of
Thin Films (Wiley, New York, N.Y., 1956). Semiconductor substrates
were obtained from Electronic Materials, Inc., Silicon Quest, Inc.
MEMS Technology Applications Center, Inc., or Crystal Specialties,
Inc.
[0094] The patterning compounds listed in Table 1 were obtained
from Aldrich Chemical Co. The solvents listed in Table 1 were
obtained from Fisher Scientific.
[0095] The AFM tips were coated with the patterning compounds as
described in Example 1 (dipping in a solution of the patterning
compound followed by drying with an inert gas), by vapor deposition
or by direct contact scanning. The method of Example 1 gave the
best results. Also, dipping and drying the tips multiple times
further improved results.
[0096] The tips were coated by vapor deposition as described in
Sherman, Chemical Vapor Deposition For Microelectronics:
Principles, Technology And Applications (Noyes, Park Ridges, N.J.,
1987). Briefly, a patterning compound in pure form (solid or
liquid, no solvent) was placed on a solid substrate (e.g., glass or
silicon nitride; obtained from Fisher Scientific or MEMS Technology
Application Center) in a closed chamber. For compounds which are
oxidized by air, a vacuum chamber or a nitrogen-filled chamber was
used. The AFM tip was position about 1-20 cm from the patterning
compound, the distance depending on the amount of material and the
chamber design. The compound was then heated to a temperature at
which it vaporizes, thereby coating the tip with the compound. For
instance, 1-octadecanethiol can be vapor deposited at 60.degree. C.
Coating the tips by vapor deposition produced thin, uniform layers
of patterning compounds on the tips and gave quite reliable results
for DPN.
[0097] The tips were coated by direct contact scanning by
depositing a drop of a saturated solution of the patterning
compound on a solid substrate (e.g., glass or silicon nitride;
obtained from Fisher Scientific or MEMS Technology Application
Center). Upon drying, the patterning compound formed a
microcrystalline phase on the substrate. To load the patterning
compound on the AFM tip, the tip was scanned repeatedly (.about.5
Hz scan speed) across this microcrystalline phase. While this
method was simple, it did not lead to the best loading of the tip,
since it was difficult to control the amount of patterning compound
transferred from the substrate to the tip.
[0098] DPN was performed as described in Example 1 using a Park
Scientific AFM, Model CP, scanning speed 5-10 Hz. Scanning times
ranged from 10 seconds to 5 minutes. Patterns prepared included
grids, dots, letters, and rectangles. The width of the grid lines
and the lines that formed the letters ranged from 15 nm to 250 nm,
and the diameters of the individual dots ranged from 12 nm to 5
micrometers.
TABLE-US-00001 TABLE 1 Patterning Potential Substrate
Compound/Solvent(s) Applications Comments and References Au
n-octadecanethiol/ Basic research Study of intermolecular forces.
acetonitrile, ethanol Langmuir, 10, 3315 (1994) Etching resist for
Etchant: KCN/O.sub.2(pH-14). microfabrication J. Vac. Sci. Tech. B,
13, 1139 (1995) dodecanethiol/ Molecular Insulating thin coating on
acetonitrile, ethanol electronics nanometer scale gold clusters.
Superlattices and Microstructures 18, 275 (1995) n-hexadecanethiol/
Etching resist for Etchant: KCN/O.sub.2(pH-14). acetonitrile,
ethanol microfabrication Langmuir, 15, 300 (1999) n-docosanethiol/
Etching resist for Etchant: KCN/O.sub.2(pH-14). acetonitrile,
ethanol microfabrication J. Vac. Sci. Technol. B, 13, 2846 (1995)
11-mercapto-1- Surface Capturing SiO.sub.2 clusters undecanol/
functionalization acetonitrile, ethanol 16-mercapto-1- Basic
research Study of intermolecular forces. hexadecanoic acid/
Langmuir 14, 1508 (1998) acetonitrile, ethanol Surface Capturing
SiO.sub.2, SnO.sub.2 clusters. J. functionalization Am. Chem. Soc.,
114, 5221 (1992) octanedithiol/ Basic research Study of
intermolecular forces. acetonitrile, ethanol Jpn. J. Appl. Phys.
37, L299 (1998) hexanedithiol/ Surface Capturing gold clusters. J.
Am. acetonitrile, ethanol functionalization Chem. Soc., 114, 5221
(1992) propanedithiol/ Basic research Study of intermolecular
forces. J. acetonitrile, ethanol Am. Chem. Soc., 114, 5221 (1992)
.alpha.,.alpha.'-p-xylyldithiol/ Surface Capturing gold clusters.
acetonitrile, ethanol functionalization Science, 272, 1323 (1996)
Molecular Conducting nanometer scale electronics junction. Science,
272, 1323 (1996) 4,4'-biphenyidithiol/ Surface Capturing gold and
CdS clusters. acetonitrile, ethanol functionalization Inorganica
Chemica Acta 242, 115 (1996) terphenyldithiol/ Surface Capturing
gold and CdS clusters. acetonitrile, ethanol functionalization
Inorganica Chemica Acta 242, 115 (1996) terphenyldiisocyanide/
Surface Capturing gold and CdS clusters. acetonitrile,
funcationalization Inorganica Chemica Acta 242, methylene chloride
115 (1996) Molecular Conductive coating on nanometer electronics
scale gold clusters. Superlattices and Microstructures, 18, 275
(1995) DNA/ Gene detection DNA probe to detect biological
water:acetonitrile (1:3) cells. J. Am. Chem. Soc. 119, 8916 (1997)
Ag n-hexadecanethiol/ Etching resist for Etchant:
Fe(NO.sub.3).sub.3(pH-6). acetonitrile, ethanol microfabrication
Microelectron. Eng., 32, 255 (1996) Al 2-mercaptoacetic acid/
Surface Capturing CdS clusters. acetonitrile, ethanol
functionalization J. Am. Chem. Soc., 114, 5221 (1992) GaAs-100
n-octadecanethiol/ Basic research Self assembled monolayer
acetonitrile, ethanol formation Etching resist for
HCl/HNO.sub.3(pH-1). microfabrication J. Vac. Sci. Technol. B, 11,
2823 (1993) TiO2 n-octadecanethiol/ Etching resist for
acetonitrile, ethanol microfabrication SiO2 16-mercapto-1- Surface
Capturing gold and CdS clusters hexadecanoic acid/
functionalization acetonitrile, ethanol octadecyltrichlorosilane
Etching resist for Etchant: HF/NH.sub.4F (pH-2). (OTS,
microfabrication Appl. Phys. Lett., 70, 1593 (1997)
CH.sub.3(CH.sub.2).sub.17SiCl.sub.3) 1.2 nm thick SAM/ hexane APTS,
3-(2- Surface Capturing nanometer scale gold
Aminoethlyamino)propyltrimethoxysilane/ functionalization clusters.
water Appl. Phys. Lett. 70, 2759 (1997)
Example 3
Atomic Force Microscopy with Coated Tips
[0099] As noted above, when an AFM is operated in air, water
condenses between the tip and surface and then is transported by
means of the capillary as the tip is scanned across the surface.
Finer et al., Langmuir 13, 6864-6868 (1997). Notably, this filled
capillary, and the capillary force associated with it,
significantly impede the operation of the AFM, especially when run
in lateral force mode. Noy et al., J. Am. Chem. Soc. 117, 7943-7951
(1995); Wilbur et al., Langmuir 11, 825-831 (1995). In air, the
capillary force can be 10 times larger than chemical adhesion force
between tip and sample. Therefore, the capillary force can
substantially affect the structure of the sample and the imaging
process. To make matters worse, the magnitude of this effect will
depend on many variables, including the relative hydrophobicities
of the tip and sample, the relative humidity, and the scan speed.
For these reasons, many groups have chosen to work in solution
cells where the effect can be made more uniform and reproducible.
Frisbie et al., Science 265, 2071-2074 (1994); Noy et al., Langmuir
14, 1508-1511 (1998). This, however, imposes a large constraint on
the use of an AFM, and solvent can affect the structure of the
material being imaged. Vezenov et al., J. Am. Chem. Soc. 119,
2006-2015 (1997). Therefore, other methods that allow one to image
in air with the capillary effect reduced or eliminated would be
desirable.
[0100] This example describes one such method. The method involves
the modification of silicon nitride AFM tips with a physisorbed
layer of 1-dodecylamine. Such tips improve one's ability to do LFM
in air by substantially decreasing the capillary force and
providing higher resolution, especially with soft materials.
[0101] All data presented in this example were obtained with a Park
Scientific Model CP AFM with a combined AFM/LFM head. Cantilevers
(model no. MLCT-AUNM) were obtained from Park Scientific and had
the following specifications: gold coated microlever, silicon
nitride tip, cantilever A, spring constant=0.05N/m. The AFM was
mounted in a Park vibration isolation chamber which had been
modified with a dry nitrogen purge line. Also, an electronic
hygrometer, placed inside the chamber, was used for humidity
measurements (.+-.5% with a range of 12.about.100%). Muscovite
green mica was obtained from Ted Pella, Inc. Soda lime glass
microscope slides were obtained from Fisher. Polystyrene spheres
with 0.23.+-.0.002 .mu.m diameters were purchased from
Polysciences, and Si.sub.3N.sub.4 on silicon was obtained from MCNC
MEMS Technology Applications Center. 1-Dodecylamine (99+%) was
purchased from Aldrich Chemical Inc. and used without further
purification. Acetonitrile (A.C.S. grade) was purchased from Fisher
Scientific Instruments, Inc.
[0102] Two methods for coating an AFM tip with 1-dodecylamine were
explored. The first method involved saturating ethanol or
acetonitrile with 1-dodecylamine and then depositing a droplet of
this solution on a glass substrate. Upon drying, the 1-dodecylamine
formed a microcrystalline phase on the glass substrate. To load the
1-dodecylamine on the AFM tip, the tip was scanned repeatedly
(.about.5 Hz scan speed) across this microcrystalline phase. While
this method was simple, it did not lead to the best loading of the
tip, since it was difficult to control the amount of 1-dodecylamine
transferred from the substrate to the tip.
[0103] A better method was to transfer the dodecylamine directly
from solution to the AFM cantilever. This method involved soaking
the AFM cantilever and tip in acetonitrile for several minutes in
order to remove any residual contaminants on the tip. Then the tip
was soaked in a .about.5 mM 1-dodecylamine/acetonitrile solution
for approximately 30 seconds. Next, the tip was blown dry with
compressed freon. Repeating this procedure several times typically
gave the best results. The 1-dodecylamine is physisorbed, rather
than chemisorbed, onto the silicon nitride tips. Indeed, the
dodecylamine can be rinsed off the tip with acetonitrile as is the
case with bulk silicon nitride. Benoit et al. Microbeam and
Nanobeam Analysis; Springer Verlag, (1996). Modification of the tip
in this manner significantly reduced the capillary effects due to
atmospheric water condensation as evidenced by several experiments
described below.
[0104] First, a digital oscilloscope, directly connected to the
lateral force detector of the AFM, was used to record the lateral
force output as a function of time. In this experiment, the force
of friction changed direction when the tip scanned left to right,
as compared with right to left. Therefore, the output of the LFM
detector switched polarity each time the tip scan direction
changed. If one or more AFM raster scans were recorded, the output
of the detector was in the form of a square wave, FIGS. 4A-B. The
height of the square wave is directly proportional to the sliding
friction of the tip on the sample and, therefore, one can compare
the forces of friction between an unmodified tip and a glass
substrate and between a modified tip and a glass substrate simply
by comparing the height of the square waves under nearly identical
scanning and environmental conditions. The tip/sample frictional
force was at least a factor of three less for the modified tip than
for the unmodified tip. This experiment was repeated on a mica
substrate, and a similar reduction in friction was observed. In
general, reductions in friction measured in this way and under
these conditions ranged from a factor of three to more than a
factor of ten less for the modified tips, depending upon substrate
and environmental conditions, such as relative humidity.
[0105] While this experiment showed that 1-dodecylamine treatment
of an AFM tip lowered friction, it did not prove that water and the
capillary force were the key factors. In another experiment, the
effects of the 1-dodecylamine coating on the capillary transport of
water was examined. Details of water transport involving unmodified
tips have been discussed elsewhere. Piner et al., Langmuir 13,
6864-6868 (1997). When an AFM tip was scanned across a sample, it
transported water to the sample by capillary action, FIG. 5A. After
scanning a 4 .mu.m.times.5 .mu.m area of a soda glass substrate for
several minutes, contiguous adlayers of water were deposited onto
the substrate and imaged by LFM by increasing the scan size. Areas
of lower friction, where water had been deposited, appeared darker
than non-painted areas, FIG. 5A. The same experiment conducted with
a tip coated with 1-dodecylamine did not show evidence of
substantial water transport, FIG. 5B. Indeed, only random
variations in friction were observed.
[0106] While these experiments showed that friction could be
reduced and the transport of water from the tip to the substrate by
capillary action could be inhibited by coating the tip with
1-dodecylamine, they did not provide information about the
resolving power of the modified tip. Mica is an excellent substrate
to evaluate this issue and, indeed, lattice resolved images could
be routinely obtained with the modified tips, demonstrating that
this modification procedure reduced the force of friction without
blunting the tip, FIG. 6A. It was impossible to determine whether
the portion of the tip that was involved in the imaging was bare or
had a layer of 1-dodecylamine on it. In fact, it is likely that the
1-dodecylamine layer had been mechanically removed from this part
of the tip exposing the bare Si.sub.3N.sub.4. In any event, the
remainder of the tip must have had a hydrophobic layer of
dodecylamine on it, since water was inhibited from filling the
capillary surrounding the point of contact, thereby reducing the
capillary effect (see above).
[0107] While the atomic scale imaging ability of the AFM was not
adversely affected by the 1-dodecylamine coating on the tip, the
above experiment did not provide useful information about the
suitability of the tip for obtaining morphology data on a larger
scale. In order to obtain such information, a sample of
monodisperse 0.23 .mu.m diameter latex spheres was imaged with both
modified and unmodified tips. Since the topography recorded by an
AFM is a convolution of the shape of the tip and the shape of the
sample, any change in the shape of the tip will be reflected in a
change in the imaged topography of the latex spheres. No detectable
difference was found in images taken with unmodified and modified
tips, respectively, FIGS. 7A-B. This shows that the shape of the
tip was not significantly changed as it would be if a metallic
coating had been evaporated onto it. Moreover, it suggests that the
1-dodecylamine coating was fairly uniform over the surface of the
tip and was sharp enough that it did not adversely affect atomic
scale imaging.
[0108] A significant issue pertains to the performance of the
modified tips in the imaging of soft materials. Typically, it is
difficult to determine whether or not a chemically-modified tip
exhibits improved performance as compared with a bare tip. This is
because chemical modification is often an irreversible process
which sometimes requires the deposition of an intermediary layer.
However, since the modification process reported herein was based
upon physisorbed layers of 1-dodecylamine, it was possible to
compare the performance of a tip before modification, after
modification, and after the tip had been rinsed and the
1-dodecylamine had been removed. Qualitatively, the
1-dodecylamine-modified tips always provided significant
improvements in the imaging of monolayers based upon alkanethiols
and organic crystals deposited onto a variety of substrates. For
example, a lattice resolved image of a hydrophilic self-assembled
monolayer of 11-mercapto-1-undecanol on a Au(111) surface was
routinely obtained with a modified tip, FIG. 6B. The lattice could
not be resolved with the same unmodified AFM tip. On this surface,
the coated tip showed a reduction in friction of at least a factor
of five by the square wave analysis (see above). It should be
noted, that the OH-terminated SAM is hydrophilic and, hence, has a
strong capillary attraction to a clean tip. Reducing the capillary
force by the modified tip allows one to image the lattice.
[0109] A second example of improved resolution involved imaging
free standing liquid surfaces, such as water condensed on mica. It
is well known that at humidities between 30 and 40 percent, water
has two distinct phases on mica. Hu et al., Science 268, 267-269
(1995). In previous work by this group, a non-contact mode scanning
polarization force microscope (SPFM) was used to image these
phases. It was found that, when a probe tip came into contact with
mica, strong capillary forces caused water to wet the tip and
strongly disturbed the water condensate on the mica. To reduce the
capillary effect so that two phases of water could be imaged, the
tip was kept .about.20 nm away from the surface. Because of this
constraint, one cannot image such phases with a contact mode
scanning probe technique. FIGS. 6C-D show images of the two phases
of water on mica recorded at 30 percent humidity with a
1-dodecylamine modified tip in contact mode. The heights of the
features (FIG. 6C) corresponded with the frictional map (FIG. 6D),
with higher features having lower friction. The quality of the
modified tip, which it is believed correlates with the uniformity
of the 1-dodecylamine layer on the tip, was important. Only well
modified tips made it possible to image the two phases of water,
while less well modified ones resulted in poorer quality images. In
fact, this was such a sensitive test that it could be used as a
diagnostic indicator of the quality of the 1-dodecylamine-modified
tips before proceeding to other samples.
[0110] In conclusion, this example describes a very simple, but
extremely useful, method for making Si.sub.3N.sub.4 AFM tips
hydrophobic. This modification procedure lowers the capillary force
and improves the performance of the AFM in air. Significantly, it
does not adversely affect the shape of the AFM tip and allows one
to obtain lattice resolved images of hydrophilic substrates,
including soft materials such as SAMs and even free-standing water,
on a solid support. The development of methodology that allows one
to get such information in air is extremely important because,
although solution cells can reduce the effect of the capillary
force, the structures of soft materials can be significantly
affected by solvent. Vezenov et al., J. Am. Soc. 119, 2006-2015
(1997). Finally, although it might be possible to make an AFM tip
more hydrophobic by first coating it with a metal layer and then
derivatizing the metal layer with a hydrophobic chemisorbed organic
monolayer, it is difficult to do so without concomitantly blunting
the AFM tip.
Example 4
Multicomponent "Dip Pen" Nanolithography
[0111] The inability to align nanoscale lithographically generated
patterns comprised of chemically distinct materials is an issue
that limits the advancement of both solid-state and molecule-based
nanoelectronics. Reed et al., Science 278, 252 (1997); Feldheim, et
al., Chem. Soc. Rev. 27, 1 (1998). The primary reasons for this
problem are that many lithographic processes: 1) rely on masking or
stamping procedures, 2) utilize resist layers, 3) are subject to
significant thermal drift problems, and 4) rely on optical-based
pattern alignment. Campbell, The Science and Engineering of
Microelectronic Fabrication (Oxford Press); Chou et al., Appl.
Phys. Lett. 67, 3114 (1995); Wang et al., Appl. Phys. Lett. 70,
1593 (1997); Jackman et al., Science 269, 664 (1995); Kim et al.,
Nature 376, 581 (1995); Schoer et al., Langmuir 13, 2323 (1997);
Whelan et al., Appl. Phys. Lett. 69, 4245 (1996); Younkin et al.,
Appl. Phys. Lett. 71, 1261 (1997); Bottomley, Anal. Chem. 70, 425R.
(1998); Nyffenegger and Penner, Chem. Rev. 97, 1195 (1997);
Berggren, et al., Science 269, 1255 (1995); Sondag-Huethorst et
al., Appl. Phys. Lett. 64, 285 (1994); Schoer and Crooks, Langmuir
13, 2323 (1997); Xu and Liu, Langmuir 13, 127 (1997); Perkins, et
al., Appl. Phys. Lett. 68, 550 (1996); Carr, et al., J. Vac. Sci.
Technol. A 15, 1446 (1997); Sugimura et al., J. Vac. Sci. Technol.
A 14, 1223 (1996); Komeda et al., J. Vac. Sci. Technol. A 16, 1680
(1998); Muller et al., J. Vac. Sci. Technol. B 13, 2846 (1995); and
Kim and M. Lieber, Science 257, 375 (1992).
[0112] With respect to feature size, resist-based optical methods
allow one to reproducibly pattern many materials, soft or
solid-state, in the >100 nm line width and spatial resolution
regime, while e-beam lithography methods allow one to pattern in
the 10-200 nm scale. In the case of soft-lithography, both e-beam
lithography and optical methods rely on resist layers and the
backfilling of etched areas with component molecules. This indirect
patterning approach compromises the chemical purity of the
structures generated and poses limitations on the types of
materials that can be patterned. Moreover, when more than one
material is being lithographically patterned, the optical-based
pattern alignment methods used in these techniques limit their
spatial resolution to approximately 100 nm.
[0113] This example describes the generation of multicomponent
nanostructures by DPN, and shows that patterns of two different
soft materials can be generated by this technique with near-perfect
alignment and 10 nm spatial resolution in an arbitrary manner.
These results should open many avenues to those interested in
molecule-based electronics to generate, align, and interface soft
structures with each other and conventional macroscopically
addressable microelectronic circuitry.
[0114] Unless otherwise specified, DPN was performed on atomically
flat Au(111) substrates using a conventional instrument (Park
Scientific CP AFM) and cantilevers (Park Scientific Microlever A).
The atomically flat Au(111) substrates were prepared by first
heating a piece of mica at 120.degree. C. in vacuum for 12 hours to
remove possible water and then thermally evaporating 30 nm of gold
onto the mica surface at 220.degree. C. in vacuum. Using atomically
flat Au(111) substrates, lines 15 nm in width can be deposited. To
prevent piezo tube drift problems, a 100 .mu.m scanner with closed
loop scan control (Park Scientific) was used for all experiments.
The patterning compound was coated on the tips as described in
Example 1 (dipping in a solution) or by vapor deposition (for
liquids and low-melting-point solids). Vapor deposition was
performed by suspending the silicon nitride cantilever in a 100 mL
reaction vessel 1 cm above the patterning compound (ODT). The
system was closed, heated at 60.degree. C. for 20 min, and then
allowed to cool to room temperature prior to use of the coated
tips. SEM analysis of tips before and after coating by dipping in a
solution or by vapor deposition showed that the patterning compound
uniformly coated the tips. The uniform coating on the tips allows
one to deposit the patterning compound on a substrate in a
controlled fashion, as well as to obtain high quality images.
[0115] Since DPN allows one to image nanostructures with the same
tool used to form them, there was the tantalizing prospect of
generating nanostructures made of different soft materials with
excellent registry. The basic idea for generating multiple patterns
in registry by DPN is related to analogous strategies for
generating multicomponent structures by e-beam lithography that
rely on alignment marks. However, the DPN method has two distinct
advantages, in that it does not make use of resists or optical
methods for locating alignment marks. For example, using DPN, one
can generate 15 nm diameter self-assembled monolayer (SAM) dots of
1,16-mercaptohexadecanoic acid (MHA) on a Au(111) faceted substrate
(preparation same as described above for atomically flat Au(111)
substrates) by holding an MHA-coated tip in contact (0.1 nN) with
the Au(111) surface for ten seconds (see FIG. 9A). By increasing
the scan size, the patterned dots are then imaged with the same tip
by lateral force microscopy (LFM). Since the SAM and bare gold have
very different wetting properties, LFM provides excellent contrast.
Wilbur et al., Langmuir 11, 825 (1995). Based upon the position of
the first pattern, the coordinates of additional patterns can be
determined (see FIG. 9B), allowing for precise placement of a
second pattern of MHA dots. Note the uniformity of the dots (FIG.
9A) and that the maximum misalignment of the first pattern with
respect to the second pattern is less than 10 nm (see upper right
edge of FIG. 9C). The elapsed time between generating the data in
FIGS. 9A and 9C was 10 minutes, demonstrating that DPN, with proper
control over environment, can be used to pattern organic monolayers
with a spatial and pattern alignment resolution better than 10 nm
under ambient conditions.
[0116] This method for patterning with multiple patterning
compounds required an additional modification of the experiment
described above. Since the MHA SAM dot patterns were imaged with an
tip coated with a patterning compound, it is likely that a small
amount of undetectable patterning compound was deposited while
imaging. This could significantly affect some applications of DPN,
especially those dealing with electronic measurements on
molecule-based structures. To overcome this problem, micron-scale
alignment marks drawn with an MHA-coated tip (cross-hairs on FIG.
10A) were used to precisely place nanostructures in a pristine area
on the Au substrate. In a typical experiment, an initial pattern of
50 nm parallel lines comprised of MHA and separated by 190 nm was
prepared (see FIG. 10A). This pattern was 2 .mu.m away from the
exterior alignment marks. Note that an image of these lines was not
taken to avoid contamination of the patterned area. The MHA-coated
tip was then replaced with an ODT-coated tip. This tip was used to
locate the alignment marks, and then precalculated coordinates
based upon the position of the alignment marks (FIG. 10B) were used
to pattern the substrate with a second set of 50 nm parallel ODT
SAM lines (see FIG. 10C). Note that these lines were placed in
interdigitated fashion and with near-perfect registry with respect
to the first set of MHA SAM lines (see FIG. 10C).
[0117] There is one unique capability of DPN referred to as
"overwriting." Overwriting involves generating one soft structure
out of one type of patterning compound and then filling in with a
second type of patterning compound by raster scanning across the
original nanostructure. As a further proof-of concept experiment
aimed at demonstrating the multiple-patterning-compound,
high-registry, and overwriting capabilities of DPN over moderately
large areas, a MHA-coated tip was used to generate three geometric
structures (a triangle, a square, and a pentagon) with 100 nm line
widths. The tip was then changed to an ODT-coated tip, and a 10
.mu.m by 8.5 .mu.m area that comprised the original nanostructures
was overwritten with the ODT-coated tip by raster scanning 20 times
across the substrate (contact force .about.0.1 nN) (dark areas of
FIG. 11). Since water was used as the transport medium in these
experiments, and the water solubilities of the patterning compounds
used in these experiments are very low, there was essentially no
detectable exchange between the molecules used to generate the
nanostructure and the ones used to overwrite on the exposed gold
(see FIG. 11).
[0118] In summary, the high-resolution,
multiple-patterning-compound registration capabilities of DPN have
been demonstrated. On an atomically flat Au(111) surface, 15 nm
patterns were generated with a spatial resolution better than 10
nm. Even on a rough surface such as amorphous gold, the spatial
resolution was better than conventional photolithographic and
e-beam lithographic methods for patterning soft materials.
* * * * *