U.S. patent application number 10/212217 was filed with the patent office on 2003-03-13 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 Hong, Seunghun, Mirkin, Chad A., Piner, Richard.
Application Number | 20030049381 10/212217 |
Document ID | / |
Family ID | 27395101 |
Filed Date | 2003-03-13 |
United States Patent
Application |
20030049381 |
Kind Code |
A1 |
Mirkin, Chad A. ; et
al. |
March 13, 2003 |
Methods utilizing scanning probe microscope tips and products
therefor or produced thereby
Abstract
The invention provides a lithographic method referred to as "dip
pen" nanolithography (DPN). DPN utilizes a scanning probe
microscope (SPM) tip (e.g., an atomic force microscope (AFM) tip)
as a "pen," a solid-state substrate (e.g., gold) as "paper," and
molecules with a chemical affinity for the solid-state substratte
as "ink." Capillary transport of molecules from the SPM tip to thee
solid substrate is used in DPN to directly write patterns
consisting of a relatively small collection of molecules in
submicrometer dimensions, making DPN useful in the facrication of a
variety of microscale and nanoscale devices. The invention also
provices substrates patterened by DPN, including submirocmeter
combinatorial arrays, and kits, devices and software for performing
DPN. The invention further provides a method of performing AFM
imaging in air. The method comprises coating an AFM tip with a
hydrophobic compound, the hydrophobic compoind being selected so
that AFM imaging perfromed using the coated AFM tipn is improved
compared to AFM imaging preformed using an uncoated AFM tip.
Finally, the invention provides AFM tips coated with the
hydrophobic compounds.
Inventors: |
Mirkin, Chad A.; (Wilmette,
IL) ; Piner, Richard; (Des Plaines, IL) ;
Hong, Seunghun; (Chicago, IL) |
Correspondence
Address: |
FOLEY AND LARDNER
SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
Northwestern University
|
Family ID: |
27395101 |
Appl. No.: |
10/212217 |
Filed: |
August 6, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10212217 |
Aug 6, 2002 |
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09866533 |
May 24, 2001 |
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10212217 |
Aug 6, 2002 |
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09477997 |
Jan 5, 2000 |
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60115133 |
Jan 7, 1999 |
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60157633 |
Oct 4, 1999 |
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60207711 |
May 26, 2000 |
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60207713 |
May 26, 2000 |
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Current U.S.
Class: |
427/402 ; 850/33;
850/55 |
Current CPC
Class: |
B82Y 10/00 20130101;
Y10S 977/853 20130101; G01Q 80/00 20130101; Y10S 977/854 20130101;
Y10S 977/88 20130101; Y10S 977/886 20130101; Y10S 977/857 20130101;
Y10S 977/856 20130101; B82B 3/00 20130101; B82Y 40/00 20130101;
Y10S 977/849 20130101; Y10S 977/855 20130101; Y10S 977/885
20130101; B05D 1/185 20130101; Y10S 977/895 20130101; B05D 1/26
20130101; Y10T 428/24802 20150115; B05D 1/007 20130101; G03F 7/0002
20130101; Y10S 977/86 20130101; Y10S 977/884 20130101; B82Y 30/00
20130101; G03F 7/165 20130101 |
Class at
Publication: |
427/402 |
International
Class: |
B05D 001/36 |
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
We claim:
1. A method of nanolithography comprising: providing a substrate;
providing a scanning probe microscope tip; coating the tip with a
patterning compound; and using the coated tip to apply the compound
to the substrate so as to produce a desired pattern.
2. The method of claim 1 wherein the substrate is gold and the
patterning compound is a protein or peptide or has 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, wherein: R.sub.1 and R.sub.2 each has the
formula X(CH.sub.2).sub.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; n is 0-30; Ar is an aryl; 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, a protein, or a ligand; and m is 0-30.
3. The method of claim 2 wherein the patterning compound has the
formula R.sub.1SH or ArSH.
4. The method of claim 3 wherein the patterning compound is
propanedithiol, hexanedithiol, octanedithiol, n-hexadecanethiol,
n-octadecanethiol, n-docosanethiol, II-mercapto-1-undecanol,
16-mercapto-1-hexadecanoic acid, .alpha.,.alpha.'-p-xylyldithiol,
4,4'-biphenyldithiol, terphenyldithiol, or DNA-alkanethiol.
5. The method of claim 1 wherein the substrate is aluminum, gallium
arsenide or titanium dioxide and the patterning compound has the
formula R.sub.1SH, wherein: R.sub.1 has the formula
X(CH.sub.2).sub.n; n is 0-30; X is --CR.sub.3, CHCH.sub.3, COOH,
CO.sub.2(CH.sub.2)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, a protein, or a ligand; and m is 0-30.
6. The method of claim 5 wherein the patterning compound is
2-mercaptoacetic acid or n-octadecanethiol.
7. The method of claim 1 wherein the substrate is silicon dioxide
and the patterning compound is a protein or peptide or has the
formula R.sub.1SH or R.sub.1SiCL.sub.3, wherein: R.sub.1 has the
formula X(CH.sub.2).sub.n; n is 0-30; X is CH.sub.3, CHCH.sub.3,
COOH, CO.sub.2(CH.sub.2).sub.mCH.su- b.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, a protein, or a ligand; and m is 0-30.
8. The method of claim 7 wherein the patterning compound is
16-mercapto-1-hexadecanoic acid, octadecyltrichlorosilane or
3-(2-aminoethylamino)-propyltrimethoxysilane.
9. The method of claim 1 wherein the substrate is oxidized gallium
arsenide or silicon dioxide and the patterning compound is a
silazane.
10. The method of claim 1 wherein the tip is coated with the
patterning compound by contacting the tip with a solution of the
patterning compound one or more times.
11. The method of claim 10 further comprising drying the tip each
time it is removed from the solution of the patterning compound,
and the dried tip is contacted with the substrate to produce the
desired pattern.
12. The method of claim 10 further comprising drying the tip each
time it is removed from the solution of the patterning compound,
except for the final time so that the tip is still wet when it is
contacted with the substrate to produce the desired pattern.
13. The method of claim 10 further comprising: rinsing the tip
after it is has been used to apply the pattern to the substrate;
coating the tip with a different patterning compound; and
contacting the coated tip with the substrate so that the patterning
compound is applied to the substrate so as to produce a desired
pattern.
14. The method of claim 13 wherein the rinsing, coating and
contacting steps are repeated using as many different patterning
compounds as are needed to make the desired pattern(s).
15. The method of claim 14 further comprising providing a
positioning system for aligning one pattern with respect to the
other pattern(s).
16. The method of claim 1 wherein the patterning compound acts as
an etching resist, and the method further comprises chemically
etching the substrate.
17. The method of claim 1 wherein a plurality of tips is
provided.
18. The method of claim 17 wherein each of the plurality of tips is
coated with the same patterning compound.
19. The method of claim 17 wherein the plurality of tips is coated
with a plurality of patterning compounds.
20. The method of claim 17 wherein each tip produces the same
pattern as the other tip(s).
21. The method of claim 20 wherein the plurality of tips comprises
an imaging tip and at least one writing tip, and each writing tip
produces the same pattern as the imaging tip.
22. The method of claim 21 wherein all of the tips are coated with
the same patterning compound.
23. The method of claim 17 wherein at least one tip produces a
pattern different than that produced by the other tip(s).
24. The method of claim 17 further comprising providing a
positioning system for aligning one pattern with respect to the
other pattern(s).
25. The method of claim 1 wherein the tip is coated with a first
patterning compound and is used to apply the first patterning
compound to some or all of a second patterning compound which has
already been applied to the substrate, the second patterning
compound being capable of reacting or stably combining with the
first patterning compound.
26. The method of claim 25 wherein the second patterning compound
has been applied to the substrate by immersing the substrate in a
solution of the compound.
27. The method of claim 1 further comprising treating the tip
before coating it with the patterning compound to enhance
physisorption of the patterning compound.
28. The method of claim 27 wherein the tip is coated with a thin
solid adhesion layer to enhance physisorption of the patterning
compound.
29. The method of claim 28 wherein the tip is coated with titanium
or chromium to form the thin solid adhesion layer.
30. The method of claim 27 wherein the patterning compound is in an
aqueous solution, and the tip is treated to make it hydrophilic in
order to enhance physisorption of the patterning compound.
31. The method of claim 1 wherein the pattern is an array of a
plurality of discrete sample areas of a predetermined shape.
32. The method of claim 31 wherein the predetermined shape is a dot
or a line.
33. The method of claim 31 wherein each of the sample areas
comprises a chemical molecule, a mixture of chemical molecules, a
biological molecule, or a mixture of biological molecules.
34. The method of claim 31 wherein each of the sample areas
comprises a type of microparticles or nanoparticles.
35. The method of claim 31 wherein the array is a combinatorial
array.
36. The method of claim 31 wherein at least one dimension of each
of the sample areas; other than depth, is less than 1 .mu.m.
37. The method of any one of claims 1-36 wherein the tip is an
atomic force microscope tip.
38. A substrate patterned by the method of an one of claims
1-36.
39. A kit for nanolithography comprising: a container holding a
patterning compound; and instructions directing that the patterning
compound be used to coat a scanning probe microscope tip and that
the coated tip be used to apply the patterning compound to a
substrate so as to produce a desired pattern.
40. The kit of claim 39 comprising a plurality of containers, each
container holding a patterning compound.
41. The kit of claim 39 or 40 further comprising one or more
additional containers, each of these containers holding a rinsing
solvent.
42. The kit of claim 39 further comprising a scanning probe
microscope tip.
43. The kit of claim 42 wherein tip is an atomic force microscope
tip.
44. The kit of claim 39 further comprising a substrate.
45. A kit for nanolithography comprising: a scanning probe
microscope tip coated with a patterning compound.
46. The kit of claim 45 wherein tip is an atomic force microscope
tip.
47. The kit of claim 45 further comprising one or more containers,
each container holding a patterning compound or a rinsing
solvent.
48. The kit of claim 45 further comprising a substrate.
49. The kit of claim 44 or 48 wherein the substrate is gold, and
the patterning compound is a protein or peptide or has 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, wherein: R.sub.1 and R.sub.2 each has the
formula X(CH.sub.2).sub.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; n is 0-30; Ar is an aryl; 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, a protein, or a ligand; and m is 0-30.
50. The kit of claim 49 wherein the patterning compound has the
formula R.sub.1SH or ArSH.
51. The kit of claim 50 wherein the patterning compound is
propanedithiol, hexanedithiol, octanedithiol, n-hexadecanethiol,
n-octadecanethiol, n-docosanethiol, 11-mercapto-1-undecanol,
16-mercapto-1-hexadecanoic acid, .alpha.,.alpha.'-p-xylyldithiol,
4,4'-biphenyldithiol, terphenyldithiol, or DNA-alkanethiol.
52. The kit of claim 44 or 48 wherein the substrate is aluminum,
gallium arsenide or titanium dioxide, and the patterning compound
has the formula R.sub.1SH, wherein: R.sub.1 has the formula
X(CH.sub.2).sub.n; n is 0-30; 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, a protein, or a ligand; and
m is 0-30.
53. The kit of claim 52 wherein the patterning compound is
2-mercaptoacetic acid or n-octadecanethiol.
54. The kit of claim 44 or 48 wherein the substrate is silicon
dioxide, and the patterning compound is a protein or peptide or has
the formula P.sub.1SH or R.sub.1SiCl.sub.3, wherein: R.sub.1 has
the formula X(CH.sub.2).sub.n; n is 0-30; 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, a protein, or a ligand; and m is 0-30.
55. The kit of claim 54 wherein the patterning compound is
16-mercapto-1-hexadecanoic acid, octadecyltrichlorosilane or
3-(2-aminoethylamino)propyltrimethoxysilane.
56. The kit of claim 44 or 48 wherein the substrate is oxidized
gallium arsenide or silicon dioxide and the patterning compound is
a silazane.
57. An atomic force microscope adapted for performing
nanolithography comprising: a sample holder adapted for receiving
and holding a substrate; and at least one well holding a patterning
compound, the well being positioned so that it will be adjacent to
the substrate when it is placed in the sample holder.
58. The microscope of claim 57 comprising a plurality of wells, at
least one well holding a patterning compound, the other well(s)
holding a patterning compound or a rinsing solvent, the wells being
positioned so that they will be adjacent the substrate when it is
placed in the sample holder.
59. An atomic force microscope adapted for performing
nanolithography comprising: a plurality of scanning probe
microscope tips; and a tilt stage adapted for receiving and holding
a sample holder, the sample holder being adapted for receiving and
holding a substrate.
60. The microscope of claim 59 wherein the plurality of scanning
probe microscope tips comprises an imaging tip and at least one
writing tip.
61. The microscope of claim 59 further comprising a plurality of
wells, each well holding a patterning compound or a rinsing
solvent, the wells being positioned so that they are adjacent to
the substrate when it is placed in the sample holder.
62. The microscope of claim 59 wherein at least one of the tips is
coated with a patterning compound.
63. The microscope of claim 62 further comprising a substrate in
the sample holder and wherein at least one of tips is contacted
with the substrate so that the patterning compound coated on the
tip is applied to the substrate so as to produce a desired
pattern.
64. The microscope of claim 63 wherein the tilt stage is adjusted
so that all of the tips are contacted with the substrate
simultaneously and each of them produces the same pattern.
65. The microscope of claim 64 wherein the plurality of scanning
probe microscope tips comprises an imaging tip and at least one
writing tip, and each writing tip produces the same pattern as the
imaging tip.
66. The microscope of claim 63 wherein the tilt stage is adjusted
so that each of the plurality of tips is contacted separately with
the substrate so that each tip produces a separate desired
pattern.
67. The microscope of any one of claims 59-66 wherein the tips are
atomic force microscope tips.
68. A submicrometer array comprising: a plurality of discrete
sample areas arranged in a pattern on a substrate, each sample area
being a predetermined shape, at least one dimension of each of the
sample areas, other than depth, being less than 1 .mu.m.
69. The array of claim 68 wherein the predetermined shape is a dot
or a line.
70. The array of claim 68 wherein each sample area comprises a
biological molecule, a mixture of biological molecules, a chemical
molecule, or a mixture of chemical molecules.
71. The array of claim 68 wherein each sample area comprises a type
of microparticles or nanoparticles.
72. The array of any one of claims 68-71 wherein the array is a
combinatorial array.
73. A method of performing atomic force microscope (AFM) imaging in
air comprising: providing an AFM tip; contacting the AFM tip with a
hydrophobic compound so that AFM imaging using the coated AFM tip
is improved compared to AFM imaging using the same tip which is
uncoated; and performing AFM imaging in air with the coated
tip.
74. The method of claim 73 wherein the hydrophobic compound has 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.
75. The method of claim 74 wherein the hydrophobic compound is
1-dodecylamine.
76. An atomic force microscope (AFM) tip coated with a hydrophobic
compound, the hydrophobic compound being selected so that AFM
imaging performed in air using the coated AFM tip is improved
compared to AFM imaging performed using the same tip which is
uncoated.
77. The tip of claim 76 which is coated with a hydrophobic compound
having the formula R.sub.4NH.sub.2 wherein: R.sub.4 is an alkyl of
the formula CH.sub.3(CH).sub.n or an aryl; and n is 0-30.
78. The tip of claim 77 which is coated with 1-dodecylamine.
79. An apparatus for depositing a compound on a substrate,
comprising: a first data collection including geometric entity data
for one or more geometric entities, wherein for a first of the
geometric entities there is: a corresponding first portion of the
first data collection, and a corresponding second data collection
of values for identifying of at least one of: the compound, the
substrate, one or more tips for depositing the compound on the
substrate, and a force of contact of at least one of said tips to a
surface of the substrate; a drawing data provider for obtaining
diffusion related information for use in drawing the first
geometric entity when said drawing data provider is supplied with
said second data collection; a pattern translator for determining
one or more drawing commands for drawing the first geometric entity
on the substrate, at least one of said drawing commands generated
using at least one of: a first value related to a time for drawing
at least a portion of the first geometric entity; wherein said at
least of the first and second values are determined using (i)
information obtained from the diffusion related information, (ii)
first information obtained from the first portion, and (iii) second
information obtained from the second data collection; a drawing
system for drawing said first geometric entity on the substrate
when provided with said one or more drawing commands, said drawing
system including a drawing tip wherein, in response to at least one
of said drawing commands, said drawing tip draws said first
geometric entity having an extent of less than one hundred
micrometers.
80. The apparatus of claim 1, wherein said drawing information
includes a diffusion constant.
81. The apparatus of claim 1, wherein at least one of: said first
value is indicative of a holding time, and said second value is
indicative of drawing speed.
82. The apparatus of claim 1 further including a computer aided
design system for obtaining said first data collection.
83. The apparatus of claim 1, wherein said drawing system includes
a scanning probe microscope.
84. The apparatus of claim 5, wherein said scanning probe
microscope includes an atomic force microscope.
85. The apparatus of claim 1, wherein said drawing data provider
includes one of: a user interface wherein a user manually enters
said drawing information, a database to which a query is input for
obtaining said drawing information, and an interpolation system for
interpolating said drawing information.
86. The apparatus of claim 1, wherein said first entity has an
extent in a range of approximately one nanometer to one hundred
micrometers.
87. A method or depositing a compound on a substrate, comprising:
first obtaining a first data collection including: (i) first
geometric entity data for a first geometric entity, and (ii) a
corresponding second data collection of one or more values for
identifying of at least one of: the compound, the substrate, one or
more tips for depositing the compound on the substrate, and (iii) a
force of contact of at least one of said tips to a surface of the
substrate; obtaining diffusion related information for use in
drawing the first geometric entity; determining one or more drawing
commands for drawing the first geometric entity on the substrate,
at least one of said drawing commands generated using at least one
of: a first value related to a time for drawing at least a portion
of the first geometric entity, and a second value related to a
drawing speed for at least a portion of the first geometric entity;
wherein said at least one of the first and second values are
determined using: (i) information obtained from the diffusion
related information, (ii) first information obtained from the first
portion, and (iii) second information obtained from the second data
collection; drawing said first geometric entity on the substrate
when provided with said one or more drawing commands, wherein, in
response to at least one of said drawing commands, a drawing tip
draws said first geometric entity having an extent of less than one
hundred micrometers.
Description
[0001] This application claims benefit of provisional applications
No. 60/115,133, filed Jan. 7, 1999, No. 60/157,633, filed Oct. 4,
1999, No. 60/207,711, filed May 26, 2000, and No. 60/207,713, filed
May 26, 2000, the complete disclosures of which are incorporated
herein by reference. This application is also a
continuation-in-part of application Ser. No. 09/477,997, filed Jan.
5, 2000, the complete disclosure of which is 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. In contrast, DPN allows one 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);
Kimet 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 (Piner 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, including combinatorial arrays, and kits, devices and
software 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 mm 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 .about.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.
[0034] FIG. 12. A schematic representation of a DPN deposition and
multi-stage etching procedure used to prepare three-dimensional
architectures in Au/Ti/Si substrates. Panel (a): Deposition of ODT
onto the Au surface of the multilayer substrate using DPN. Panel
(b): Selective Au/Ti etching with ferri/ferrocyanide-based etchant.
Panel (c): Selective Ti/SiO.sub.2 etching and Si passivation with
HF. Panel (d): Selective Si etching with basic etchant and
passivation of Si surface with HF. Panel (e): Removal of residual
Au and metal oxides with aqua regia and passivation of Si surface
with HF.
[0035] FIG. 13A-C. Nanometer scale pillars prepared according to
FIG. 12, Panels a-d. FIGS. 13A: AFM topography image after
treatment of wafer patterned with 4 dots with 2 second deposition
time. Pillar height is 55 nm. The identification letter and top
diameter (nm) are the following: A, 65; B, 110; C, 75; D, 105.
Recorded at a scan rate of 2 Hz. FIG. 13B: The AFM topography image
of a pillar on the same chip. Pillar height is 55 nm. Recorded at a
scan rate of 1 Hz. FIG. 13C: The cross-sectional trace of the AFM
topography image through the pillar diameter.
[0036] FIGS. 14A-C. Nanometer scale lines prepared according to
FIG. 12, Panels a-d. FIG. 14A: AFM topography image after treatment
of wafer patterned with 3 lines of ODT at a rate of 0.4
.mu.m/second. Line height is 55 nm. Recorded at a rate of 0.5 Hz.
FIG. 14B: AFM topography image of a line on the same chip. Line
height is 55 nm. Recorded at a rate of 0.5 Hz. FIG. 14C:
Cross-sectional topography trace of the line.
[0037] FIGS. 15A-C. Pillars prepared according to FIG. 12, Panels
a-d. FIG. 15A: An ODT-coated AFM tip was held in contact with the
surface for various times to generate ODT dots of increasing size.
Three-dimensional features with a height of 80 nm were yielded
after etching. The identification letter, time of ODT deposition
(seconds), estimated diameter of ODT dot (nm), top diameter after
etching (nm), and base diameter after etching (nm) are the
following: A, 0.062, 90, 147, 514; B, 0.125, 140, 176, 535; C,
0.25, 195, 253, 491; D, 0.5, 275, 314, 780; E, 1, 390, 403, 892; F,
2, 555, 517, 982; G, 4, 780, 770, 1120; H, 8,, 1110, 1010, 1430; I,
16, 1565, 1470, 1910. FIG. 15B: SEM of same pillars. FIG. 15C: Top
diameter plotted as a function of ODT deposition time.
[0038] FIGS. 16A-B. Lines prepared according to FIG. 12, Panels
a-d. FIG. 16A: The AFM topography image of lines on the same chip
as used for preparation of the pillars shown in FIG. 15. An
ODT-coated AFM tip was used to generate lines on the surface with
various speeds to generate various sized ODT lines. The
three-dimensional features shown in FIG. 16A with a height of 80 nm
were yielded after etching. The identification letter, speed of ODT
deposition (.mu.m/second), top line width after etching (nm), and
base width are the following: A, 2.8, 45, 45, 213; B, 50, 2.4, 70,
402; C, 60, 2.0, 75, 420; D, 1,6, 75, 90, 430; F, 1.2, 100, 120,
454; F, 150, 0.8, 150, 488; G, 0.4, 300, 255, 628, H, 0.2, 600,
505, 942. FIG. 16B: SEM of the same lines.
[0039] FIG. 17: Diagram illustrating the components of a DPN
nanoplotter and parallel writing.
[0040] FIG. 18: Diagram of an array of AFM tips for parallel
writing.
[0041] FIG. 19: ODT nanodot and line features on Au generated by
the same tip but under different tip-substrate contact forces.
There is less than 10% variation in feature size.
[0042] FIGS. 20A-B: Parallel DPN writing using two tips and a
single feedback system. FIG. 20A: Two nearly identical ODT patterns
generated on Au in parallel fashion with a two pen cantilever. FIG.
20B: Two nearly identical patterns generated on Au in parallel
fashion with a two pen cantilever, each pen being coated with a
different ink. The pattern on the left is generated from an
MHA-coated tip and exhibits a higher lateral force than the Au
substrate. The pattern on the right was generated with an ODT
coated tip and exhibits a lower lateral force than the Au
substrate.
[0043] FIGS. 21A-C: Nanoplotter-generated patterns which consist of
features comprised of two different inks, ODT and MHA. The patterns
were generated without removing the multiple-pen cantilever from
the instrument. FIG. 21A: Two-ink, cross-shaped pattern (ODT
vertical lines and MHA horizontal lines) with an MHA dot in the
center of the pattern (note the circular shape of the dot). FIG. 21
B: A molecular cross-shaped corral made of ODT. MHA molecules
introduced into the center of the corral diffuse from the center of
the corral but are blocked when they reach the 80 nm-wide ODT
walls. Note the convex shape of the MHA ink within the molecular
corral due to the different wetting properties of the gold
substrate and hydrophobic corral. FIG. 21 C: A molecular
cross-shaped corral, where the horizontal lines are comprised of
MHA and the vertical lines are comprised of ODT. Note that the MHA,
which is introduced in the center of the corral, diffuses over the
walls of the corral comprised of MHA but remains confined within
the walls comprised of ODT. Also, note that the MI-IA structure
within the corral assumes a concave shape where the sidewalls are
made of MHA (horizontal black arrow) and a convex shape where the
sidewalls are made of ODT (vertical black arrow).
[0044] FIG. 22: Eight identical patterns generated with one imaging
tip (which uses a feedback system) and seven writing tips (passive;
do not use feedback systems separate from that of the imaging tip),
all coated with ODT molecules.
[0045] FIG. 23: A schematic representation of the DPN-based
particle organization strategy.
[0046] FIGS. 24A-C: Patterns generated on gold thin film by DPN,
imaged by lateral force microscopy (MHA=light areas, ODT=dark
areas). MHA dots [diameters 540 (FIG. 24 A), 750 (FIG. 24B), and
240 nm (FIG. 24C), center-to-center distance 2 .mu.m] deposited by
holding the AFM tip at a series of x,y coordinates (5, 10, and 15
seconds). Scale bars represent 6 .mu.m.
[0047] FIG. 25: Optical micrograph of particle arrays on a
MHA-patterned substrate. Scale bar represents 20 .mu.m.
[0048] FIG. 26: In situ optical micrograph of 1.0 .mu.m diameter
amine-modified polystyrene particles organized into a square array
with a lattice constant of 2 .mu.m. Note the dark fuzzy dots, which
are particles in solution that have not reacted with the template
(white arrows). Scale bar represents 6 .mu.m.
[0049] FIGS. 27A-B: Two regions of a gold substrate with 190 nm
amidine-modified polystyrene particles selectively organized on MHA
regions of the patterned surface, imaged by intermittent-contact
AFM. FIG. 27A-single particle array formed on 300 nm MHA dots. FIG.
27B single particle array formed on 700 nm diameter MHA dots. Also,
note that the AFM tip in some case drags the particles from their
preferred locations.
[0050] FIG. 28A: Block diagram illustrating DPN software.
[0051] FIG. 28B: Flow chart illustrating pattern translator
subroutine of DPN software.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
[0052] DPN utilizes a scanning probe microscope (SPM) tip. As used
herein, the phrases "scanning probe microscope tip" and "SPM tip"
are used to mean 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, including devices made
especially for DPN using the guidelines provided herein. Many SPM
tips are available commercially (e.g., from Park Scientific,
Digital Instruments, Molecular Imaging, Nanonics Ltd. and
Topometrix. Alternatively, SPM tips can be made by methods well
known in the art. For instance, SPM tips can be made by e-beam
lithography (e.g., a solid tip with a hole bored in it can be
fabricated by e-beam lithography).
[0053] Most preferably, the SPM tip is an AFM tip. Any AFM tip can
be used, and suitable AFM tips include those that are available
commercially from, e.g., Park Scientific, Digital Instruments and
Molecular Imaging. Also preferred are NSOM tips usable in an AFM.
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. STM tips usable in an AFM are also suitable for DPN,
and such tips can be fabricated (see, e.g. Giessibl et al.,
Science, 289, 422 (2000)) or can be obtained commercially (e.g.,
from Thermomicroscopes, Digital Instruments, or Molecular
Imaging).
[0054] The tip is also preferably 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 acetonitriles 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.
[0055] 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, Agl,
AgBr, Hgl2, 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).
[0056] 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.
[0057] 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:
[0058] 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;
[0059] 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;
[0060] c. Compounds of the formula R.sub.1NC, R.sub.1SH,
R.sub.1SSR.sub.2, or R.sub.1SR.sub.2can be used to pattern platinum
substrates;
[0061] d. Compounds of the formula R.sub.1SH can be used to pattern
aluminum, TiO.sub.2SiO.sub.2, GaAs and InP substrates;
[0062] 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;
[0063] f. Compounds of the formula R.sub.1COOH or
R.sub.1CONHR.sub.2 can be used to pattern metal oxide
substrates;
[0064] 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;
[0065] 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;
[0066] i. Compounds of the formula R.sub.1COOH can be used to
pattern aluminum, copper, silicon and platinum substrates;
[0067] 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;
[0068] k. Proteins and peptides can be used to pattern, gold,
silver, glass, silicon, and polystyrene; and
[0069] l. Silazanes can be used to pattern SiO.sub.2 and oxidized
GaAs.
[0070] In the above formulas:
[0071] R.sub.1 and R.sub.2 each has the formula X(CH.sub.2).sub.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;
[0072] R.sub.3 has the formula CH.sub.3(CH.sub.2).sub.n;
[0073] n is 0-30;
[0074] Ar is aryl;
[0075] 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
[0076] m is 0-30.
[0077] 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
oligonucleotide 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 Chemistry 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 la990467r
(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 nitrites 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 nitrites to gold);
Solomun et al. Ber. Bunsen-Ges. Phys. Chem., 95,95-8(1991)
(attachment of nitrites 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 phosphines 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 siher); 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, 24pp (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); Bansal 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, 1& 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); Pereira et al., J.
Mater. Chem., 10, 259 (2000) (attachment of silazanes to
SiO.sub.2); Pereira et al., J. Mater. Chem., 10, 259 (2000)
(attachment of silazanes to SiO.sub.2); Dammel, Diazonaphthoquinone
Based Resists (1.sup.st ed., SPIE Optical Engineering Press,
Bellingham, Wash., 1993) (attachment of silazanes to SiO.sub.2);
Anwander et al., J. Phys. Chem. B, 104, 3532 (2000) (attachment of
silazanes to SiO.sub.2); Slavov et al., J. Phys. Chem., 104, 983
(2000) (attachment of silazanes to SiO.sub.2).
[0078] 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 arylthiols on a variety of
substrates and trichlorosilanes on SiO.sub.2 substrates (see
Examples 1 and 2).
[0079] 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.
[0080] 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
amicrocrystalline 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.
[0081] 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.
[0082] 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).
[0083] To perform DPN, the coated tip is used to apply a patterning
compound to a substrate so as to form a desired pattern. The
pattern may be any pattern and may be simple or complex. For
instance, the pattern may be a dot, a line, a cross, a geometric
shape (e.g. a triangle, square or circle), combinations of two or
more of the foregoing, combinatorial arrays (e.g., a square array
of rows and columns of dots), electronic circuits, or part of, or a
step in, the formation of a three-dimensional structure.
[0084] A transport medium is preferably used in DPN since, as
presently understood, the patterning compound is transported to the
substrate by capillary transport. 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. The tip may be
actually touching the substrate, but it need not be. The tip only
needs to be close enough to the substrate so that a 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. The
possibility that the patterning compound can be deposited on the
substrate without the use of a transport medium has not been
completely ruled out, although it seems highly unlikely. Even under
conditions of low, or even no humidity, there is likely some water
on the substrate which could function as the transport medium.
[0085] DPN is performed using an AFM or a device performing similar
functions and having similar properties, including devices
developed especially for performing DPN using the guidelines
provided herein, using techniques that are conventional and well
known in AFM microscopy. Briefly, the substrate is placed in the
sample holder of the device, the substrate is contacted with the
SPM tip(s) coated with the patterning compound(s), and the
substrate is scanned to pattern it with the patterning compound(s).
An AFM can be operated in several modes, and DPN can be performed
when the AFM or similar device is operated in any of these modes.
For instance, DPN can be performed in (1) contact (constant force)
mode wherein the tip is maintained in contact with (touching) the
substrate surface, (2) non-contact (dynamic) mode wherein the tip
is vibrated very close to the substrate surface, and/or (3)
intermittent contact (tapping) mode which is very similar to the
non-contact mode, except that the tip is allowed to strike (touch)
the surface of the substrate.
[0086] Single tips can be to write a pattern utilizing an AFM or
similar device. 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: (1) removing a first tip
coated with a first patterning compound and replacing it with
another tip coated with a different patterning compound; or (2)
rinsing the first tip coated with the first patterning compound so
as to remove the patterning compound from the tip and then coating
the tip with a different patterning compound. Suitable solvents for
rinsing tips to remove patterning compounds are those solvents in
which the patterning compound is soluble. Preferably, the rinsing
solvent is the solvent in which the patterning compound is most
soluble. Rinsing of tips can be accomplished by simply dipping the
tip in the rinsing solvent.
[0087] Alternatively, a plurality of tips can be used in a single
AFM or similar 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., Example 6 below, U.S.
Pat. Nos. 5,630,923, and 5,666,190, Lutwyche et al., Sens.
Actuators A, 73:89 (1999), Vettiger et al., Microelectron Eng.,
46:11(1999), Minne et al., Appl. Phys. Left., 73:1742 (1998), and
Tsukamoto et al., Rev. Sci. Instrum., 62:1767 (1991) which describe
devices comprising multiple cantilevers and tips for patterning a
substrate). One or more of the plurality of tips can be rinsed as
described above for single tips, if desired, to change the
patterning compound coated on the tip(s).
[0088] The AFM or similar device used for DPN preferably comprises
at least one micron-scale well positioned so that the well(s) will
be adjacent the substrate when the substrate is placed in the
sample holder. Preferably the AFM or similar device comprises a
plurality of wells holding a plurality of patterning compounds or
holding at least one patterning compound and at least one rinsing
solvent. "Well" is used herein to mean any container, device, or
material that can hold a patterning compound or rinsing solvent and
includes depressions, channels and other wells which can be
prepared by microfabrication (e.g, the same processes used to
fabricate microelectronic devices, such as photolithograpy; see,
e.g., PCT application WO 00/04390). The wells may also simply be
pieces of filter paper soaked in a patterning compound or rinsing
solvent. The wells can be mounted anywhere on the AFM or similar
device which is adjacent the substrate and whereby they can be
addressed by the SPM tip(s), such as on the sample holder or
translation stage.
[0089] 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.
[0090] DPN can also be used in a nanoplotter format by having a
series of wells containing a plurality of different patterning
compounds and rinsing solvents adjacent the substrate. One or more
tips can be used. When a plurality of tips is used, the tips can be
used serially or in parallel to produce patterns on the
substrate.
[0091] In a nanoplotter format using a single tip, the tip is
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. The tip is then rinsed by dipping it in a well
containing a rinsing solvent or a series of such wells. The rinsed
tip is then dipped into another well to be coated with a second
patterning compound and is 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 patterning compounds, applying a pattern to the substrate, and
rinsing the tip, can be repeated as many times as desired, and the
entire process can be automated using appropriate software.
[0092] A particularly preferred nanoplotter format is described in
Example 6 and illustrated in FIGS. 17 and 18. In this preferred
format, a plurality of AFM tips are attached to an AFM. A
multiple-tip array can be fabricated by simply physically
separating an array of the desired number of cantilevers from a
commercially-available wafer block containing a large number of
individual cantilevers, and this array can be used as a single
cantilever on the AFM. The array can be attached to the AFM tip
holder in a variety of ways, e.g. with epoxy glue. Of course,
arrays of tips of any spacing or configuration and adapted for
attachment to an AFM tip holder can be microfabricated by methods
known in the art. See, e.g., Minne et al., Applied Physics Letters,
72:2340 (1998). The plurality of tips in the array can be employed
for serial or parallel DPN. When the plurality of tips is used for
parallel DPN, only one of the tips needs to be connected to a
feedback system (this tip is referred to as the "imaging tip"). The
feedback system is a standard feedback system for an AFM and
comprises a laser, photodiode and feedback electronics. The
remaining tips (referred to as "writing tips") are guided by the
imaging tip (i.e., all of the writing tips reproduce what occurs at
the imaging tip in passive fashion). As a consequence, all of the
writing tips will produce the same pattern on the substrate as
produced by the imaging tip. Of course, each writing tip may be
coated with a patterning compound which is the same or different
than that coated on the imaging tip or on the other writing tips,
so that the same pattern is produced using the same patterning
compound or using different patterning compounds. When serial DPN
is employed, each of the tips used in sequence must be connected to
a feedback system (simultaneously or sequentially). The only
adaptation of the AFM necessary to provide for a choice of serial
or parallel DPN is to add a tilt stage to the AFM. The tilt stage
is adapted for receiving and holding the sample holder, which in
turn is adapted for receiving and holding the substrate. Tilt
stages are included with many AFM's or can be obtained commercially
(e.g., from Newport Corp.) and attached to the AFM according to the
manufacturer's instructions. The AFM preferably also comprises a
plurality of wells located adjacent the substrate and so that the
AFM operator can individually address and coat the tips with
patterning compounds or rinse the tips with rinsing solvents. Some
AFM's are equipped with a translation stage which can move very
large distances (e.g., the M5 AFM from Thermomicroscopes), and the
wells can be mounted on this type of translation stage. For inking
or rinsing, a well is moved below an AFM tip by the translation
stage and, then, the tip is lowered by a standard coarse approach
motor until it touches the ink or solvent in the well. The tip is
held in contact with the ink or solvent in order to coat or rinse
the tip. The wells could also be mounted on the sample holder or
tilt stage.
[0093] DPN can also be used to apply a second patterning compound
to a first patterning compound which has already been applied to a
substrate. The first patterning compound can be applied to the
substrate by DPN, microcontact printing (see, e.g, Xia and
Whitesides, Angew. Chem. Ind Ed., 37, 550-575 (1998); James et al.,
Langmuir, 14, 741-744 (1998); Bernard et al., Langmuir, 14,
2225-2229 (1998); Huck et al., Langmuir, 15, 6862-6867 (1999)), by
self-assembly of a monolayer on a substrate immersed in the
compound (see, e.g. Ross et al., Langmuir, 9, 632-636 (1993);
Bishop and Nuzzo, Curr. Opinion in Colloid & Interface Science,
1, 127-136 (1996); Xia and Whitesides, Angew. Chem. Ind. Ed., 37,
550-575 (1998);Yan et al., Langmuir, 15, 1208-1214 (1999); Lahiri
et al., Langmuir, 15, 2055-2060 (1999); Huck et al., Langmuir, 15,
6862-6867 (1999)), or any other 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.
[0094] 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.
[0095] DPN can also be used in conjunction with wet (chemical)
etching techniques. In particular, an SPM tip can be used to
deliver a patterning compound to a substrate of interest in a
pattern of interest, all as described above, and the patterning
compound functions as an etching resist in one or more subsequent
wet etching procedures. The patterning compounds can be used to
pattern the substrate prior to any etching or after one or more
etching steps have been performed to protect areas exposed by the
etching step(s). The wet etching procedures and materials used in
them are standard and well known in the art. See, e.g., Xia et al.,
Angew. Chem. Int. Ed., 37, 550 (1998); Xia et al., Chem. Mater., 7,
2332 (1995); Kumar et al., J. Am. Chem. Soc., 114, 9188-9189
(1992); Seidel et al., J. Electrochem. Soc., 137, 3612 (1990). Wet
etching procedures are used for, e.g., the preparation of
three-dimensional architectures on or in substrates (e.g., Si
wafers) of interest. See, e.g., Xia et al., Angew. Chem. Int. Ed,
37, 550 (1998); Xia et al., Chem. Mater., 7, 2332 (1995). After
etching, the patterning compound may be retained on the substrate
or removed from it. Methods of removing the patterning compounds
from the substrates are well known in the art. See, e.g., Example
5.
[0096] 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).
[0097] 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.
[0098] 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.
[0099] 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 (Piner 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.
[0100] 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).
[0101] 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, microarialytical
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).
[0102] DPN, particularly parallel DPN, should also be especially
useful for the preparation of arrays, particular combinatorial
arrays. An "array" is an arrangement of a plurality of discrete
sample areas in a pattern on a substrate. The sample areas may be
any shape (e.g., dots, circles, squares or triangles) and maybe
arranged in any pattern (e.g., rows and columns of discrete sample
areas). Each sample area may contain the same or a different sample
as contained in the other sample areas of the array. A
"combinatorial array" is an array wherein each sample area or a
small group of replicate sample areas (usually 2-4) contain(s) a
sample which is different than that found in other sample areas of
the array. A "sample" is a material or combination of materials to
be studied, identified, reacted, etc.
[0103] DPN will be particularly useful for the preparation of
combinatorial arrays on the submicrometer scale. An "array on the
submicrometer scale" means that at least one of the dimensions
(e.g, length, width or diameter) of the sample areas, excluding the
depth, is less than 1 .mu.m. At present, DPN can be used to prepare
dots that are 10 nm in diameter. With improvements in tips (e.g.,
sharper tips), it should be possible to produce dots that approach
1 nm in diameter. Arrays on a submicrometer scale allow for faster
reaction times and the use of less reagents than the currently-used
microscale (i.e., having dimensions, other than depth, which are
1-999 .mu.m) and larger arrays. Also, more information can be
gained per unit area (i.e., the arrays are more dense than the
currently-used micrometer scale arrays). Finally, the use of
submicrometer arrays provides new opportunities for screening. For
instance, such arrays can be screened with SPM's to look for
physical changes in the patterns (e.g., shape, stickiness, height)
and/or to identify chemicals present in the sample areas, including
sequencing of nucleic acids (see below).
[0104] Each sample area of an array contains a single sample. For
instance, the sample may be a biological material, such as a
nucleic acid (e.g., an oligonucleotide, DNA, or RNA), protein or
peptide (e.g., an antibody or an enzyme), ligand (e.g., an antigen,
enzyme substrate, receptor or the ligand for a receptor), or a
combination or mixture of biological materials (e.g., a mixture of
proteins). Such materials may be deposited directly on a desired
substrate as described above (see the description of patterning
compounds above). Alternatively, each sample area may contain a
compound for capturing the biological material. See, e.g. PCT
applications WO 00/04382, WO 00/04389 and WO 00/04390, the complete
disclosures of which are incorporated herein-by reference. For
instance, patterning compounds terminating in certain functional
groups (e.g., --COOH) can bind proteins through a functional group
present on, or added to, the protein (e.g., --NH.sub.2). Also, it
has been reported that polylysine, which can be attached to the
substrate as described above, promotes the binding cells to
substrates. See James et al., Langmuir, 14, 741-744 (1998). As
another example, each sample area may contain a chemical compound
(organic, inorganic and composite materials) or a mixture of
chemical compounds. Chemical compounds may be deposited directly on
the substrate or may be attached through a functional group present
on a patterning compound present in the sample area. As yet another
example, each sample area may contain a type of microparticles or
nanoparticles. See Example 7. From the foregoing, those skilled in
the art will recognize that a patterning compound may comprise a
sample or may be used to capture a sample.
[0105] Arrays and methods of using them are known in the art. For
instance, such arrays can be used for biological and chemical
screenings to identify and/or quantitate a biological or chemical
material (e.g., immunoassays, enzyme activity assays, genomics, and
proteomics). Biological and chemical libraries of
naturally-occurring or synthetic compounds and other materials,
including cells, can be used, e.g., to identify and design or
refine drug candidates, enzyme inhibitors, ligands for receptors,
and receptors for ligands, and in genomics and proteomics. Arrays
of microparticles and nanoparticles can be used for a variety of
purposes (see Example 7). Arrays can also be used for studies of
crystallization, etching (see Example 5), etc. References
describing combinatorial arrays and other arrays and their uses
include U.S. Pat. Nos. 5,747,334, 5,962,736, and 5,985,356, and PCT
applications WO 96/31625, WO 99/31267, WO 00/04382, WO 00/04389, WO
00/04390, WO 00/36 136, and WO 00/46406.
[0106] Results of experiments performed on the arrays of the
invention can be detected by conventional means (e.g.,
fluorescence, chemiluminescence, bioluminescence, and
radioactivity). Alternatively, an SPM can be used for screening
arrays. For instance, an AFM can be used for quantitative imaging
and identification of molecules, including the imaging and
identification of chemical and biological molecules through the use
of an SPM tip coated with a chemical or biomolecular identifier.
See Frisbie et al., Science, 265,2071-2074 (1994); Wilbur et al.,
Langmuir, 11, 825-831 (1995); Noy et al., J. Am. Chem. Soc., 117,
7943-7951(1995); Noy et al., Langmuir, 14, 1508-1511 (1998); and
U.S. Pat. Nos. 5,363,697, 5,372,93, 5,472,881 and 5,874,668, the
complete disclosures of which are incorporated herein by
reference.
[0107] The present invention also includes novel components for
more precisely depositing patterns on a substrate by DPN. In
particular, the present invention includes a component that
receives as input dot sizes and line widths of the patterning
compound to be deposited on the substrate, and subsequently
determines the corresponding parameter values that can be used in
controlling the lower level software and hardware that deposits the
substance on the substrate, e.g., such lower level software and
hardware includes AFM systems. That is, since such lower level
software and hardware (also denoted herein as AFM software and AFM
hardware) typically are controlled by inputs such as "holding time"
or stationarily depositing a dot of a desired size (e.g.,
diameter), and/or substrate drawing speed for depositing a line
having a desired line width, the present invention includes a
component for translating between: (a) dot size and line width, and
(b) holding time and drawing speed, respectively. Moreover, since
it is has been determined that dot size and line width are each a
function of the diffusion rate of the patterning compound onto the-
substrate, the component for translating (also denoted a "pattern
translator" or merely "translator" herein) translates between (a)
and (b) above by using such diffusion rates. In particular, the
applicants have determined that:
[0108] (i) dot size may be determined according to the following
equation:
R={square root}{square root over (C*t/.pi.)},
[0109] where R is the radius of the dot, t is the holding time, and
C is the diffusion constant, wherein C is, in turn, determined by
the tip characteristics, the substrate, the patterning compound,
and the contact force of the tip against the substrate; and
[0110] (ii) line width may be determined according to the following
equation:
W=C/s,
[0111] wherein W is the line width, s is the tip sweeping (e.g.,
drawing) speed, and C is as described above
[0112] To more fully describe the components for performing the
precision DPN of the present invention, reference is made to FIG.
28A which is a high level diagram of the DPN system 2004 of the
present invention. Accordingly, this system includes a DPN geometry
engine 2008 which provides a user interactive DNP application
software components for allowing a user to interactively design DPN
patterns. In one embodiment, the DNP application components are
provided on a WINDOWS 2000 platform by Microsoft Corp. More
specifically, the DPN geometry engine 2008 includes the following
modules:
[0113] To more fully describe the components for performing the
precision DPN of the present invention, reference is made to FIG.
28A which is a high level diagram of the DPN system 2004 of the
present invention. Accordingly, this system includes a DPN geometry
engine 2008 which provides a user interactive DNP application
software components for allowing a user to interactively design DPN
patterns. In one embodiment, the DNP application components are
provided on a WINDOWS 2000 platform by Microsoft Corp. More
specifically, the DPN geometry engine 2008 includes the following
modules:
[0114] (a) A computer aided design system 2012 (CAD) for generating
at least two dimensional patterns.
[0115] (b) A user interface 2016 for interacting with the computer
aided design system, and for supplying information related
specifically to the DPN process to be performed, such as, the
identifications of the substrate, and the patterning compound to be
deposited. Additionally, a user may be able to input tip
characteristics such as tip shape, and tip materials, as well as an
expected tip contact force against substrate. Note that the user
interface 2016 may provide graphical presentations to the user's
display 2020. Alternatively, the user interface may receive input
from a non-interactive source such a networked database (not
shown). In one embodiment, the user may have multiple concurrent
window presentations of his/her pattern or design.
[0116] (c) A DPN runtime parameter storage 2024 for storing the DPN
specific parameters such as the identification of the substrate and
patterning compound, the tip characteristics, and contact force as
in (b) immediately above.
[0117] Patterns are output from the CAD 2012 to the pattern
translator 2028 for translating into specifications of dots and
piecewise linear shapes that can then be output to the drawing
system 2030 which, e.g., may be an atomic force microscope system.
In particular, this output is provided to the AFM software drivers
2032, wherein as mentioned above these drivers accept commands
having values of holding time and drawing speed rather than dot
size and line width. Additionally, the pattern translator 2028 also
receives input from the DPN runtime parameter storage 2024
providing the parameter values identified in (c) above. Note that
upon receiving the inputs from the CAD 2012 and the parameter
storage 2024, the pattern translator 2028 may query a diffusion
calibration database/expert system 2036 for the diffusion
constant(s) C as described hereinabove. That is, the pattern
translator 2028 uses the parameter values obtained from the
parameter storage 2024 to query the diffusion calibration
database/expert system 2036 for the corresponding diffusion
constant(s) C that are to be applied to corresponding input from
the CAD 2012. Subsequently, the pattern translator 2028 generates
AFM commands for output to the AFM software drivers 2032, wherein
each of the AFM commands is typically one of the following tip
movement commands:
[0118] (a) Keep the tip away from the substrate surface.
[0119] (b) Hold the tip in contact with the substrate surface at a
fixed position for a given time (t) with a given force.
[0120] (c) Move the tip, while in contact with the substrate, in a
line from a first point to a second point at a given (fixed or
variable) speed.
[0121] Subsequently, the AFM software drivers 2032 direct the AFM
hardware 2040 to apply the patterning compound to the substrate
according to the commands received by the AFM software drivers
2032. Note that, for at least some of the AFM commands, the
corresponding tip movement is in a range of approximately one
nanometer to one hundred micrometers. However, dots provided by the
present invention may be approximately one nanometer. Moreover, it
is within the scope of the present invention that the AFM software
drivers 2032 and the AFM hardware 2040 may utilize multiple drawing
tips for drawing on the substrate. In particular, each drawing tip
may use a different patterning compound (e.g., different ink). Note
that the AFM software drivers 2032 may generate the tip controls
for which of the multiple tips to use at any given time during a
drawing of a pattern by the drawing system 2030.
[0122] Note that the AFM software drivers 2032 can be commercially
obtained from Thermomicroscopes, 9830 S. 51st Street,. Suite A 124
Phoenix, Ariz. 85044. Additionally, the AFM hardware can be
obtained from Thermomicroscopes or one or more of the following
companies: Veeco_Inc., 112 Robin Hill Road, Santa Barbara, Calif.
93117, or Molecular Imaging Inc., 1171 Borregas Avenue, Sunnyvale,
Calif. 94089.
[0123] Additionally, note in an alternative embodiment, the
diffusion rates may be empirically determined by the user, and
accordingly, the diffusion calibration database/expert system 2036
maybe unnecessary. Instead the user may enter the diffusion rates,
e.g., via the user interface 2016.
[0124] In FIG. 28B a high level flowchart is provided of the steps
performed by the pattern translator 2028. In step 2054, the pattern
translator 2028 receives a design (CAD) file from the CAD 2012. In
step 2058, the pattern translator 2028 retrieves all corresponding
DPN parameters for the DPN runtime parameter storage 2024. Note
that, in one embodiment, there may be different such parameter
values for different geometric data entities in the CAD file.
Additionally, note that in another embodiment, the DPN parameter
values may be provided in the CAD file and associated with their
corresponding geometric entities. Further, in a simple case where
such DPN parameter values are the same for all geometric entities,
the DPN parameter values may occur in the CAD file only once
wherein this occurrence is applicable to all geometric entities
therein. Following this, in step 2062, a first geometric entity in
the design file is obtained (denoted herein as "G"). Thus, in step
2066, the corresponding DPN parameter values are determined for G.
Subsequently, in step 2070, the diffusion constant, C.sub.0, is
obtained from the diffusion calibration database/expert system
2036. Note that as this database's name implies, it may be
substantially a database (e.g., a relational database) that
contains, e.g., a table associating a dot size, a patterning
compound, a substrate, tip characteristics, and a contact force
with a desired holding time for obtaining the dot size for the
patterning compound on the substrate when the tip has the tip
characteristics and the tip contacts the substrate surface with the
contact force. Similarly, such a database will have a table
associating a line width, a patterning compound, a substrate, tip
characteristics, and a contact force with a desired holding time
for obtaining the line width for a line of the patterning compound
on the substrate when the tip has the tip characteristics and the
tip contacts the substrate surface with the contact force. For
example, the following is an illustration of entries in such a
table:
1 Patterning Diffusion Compounds Substrate Tip Contact force
constant 1-octadecanethiol gold Microlever 1 nano newton 0.08 A
mm.sup.2/sec 16-mercapto- gold Microlever 1 nano newton 0.04
hexadecanoic acid A mm.sup.2/sec Silazane gold Microlever 1 nano
newton 0.05 A mm.sup.2/sec Silazane gold Microlever 1 nano newton
0.03 A mm.sup.2/sec
[0125] Note, however, in some embodiments, such tables may be very
large and/or not all combinations will have been previously
determined (i.e., calibrated). Accordingly, where the invention
embodiment is used with, e.g., various combinations of patterning
compounds (e.g., different inks, or etching mask substances),
and/or on various substrates, and/or where various types of tips
may be used, the diffusion calibration database/expert system 2036
may intelligently compute, infer or interpolate a likely holding
time and/or drawing speed. For example, a rule based expert system
may be one embodiment of the diffusion calibration database/expert
system 2036 for determining a likely diffusion constant.
Additionally, note that when such a new a dot size and/or line
width is verified for a particular patterning compound, substrate,
tip characteristics, and contact force, then such values may be
associated and stored for subsequent use by the diffusion
calibration database/expert system 2036.
[0126] In another alternative embodiment, instead of storing the
diffusion constant, the holding times and drawing speeds may be
associated with dot size and line width as well as the patterning
compound, substrate, tip characteristics, and contact force.
[0127] Referring again to FIG. 28B, in step 2074, the diffusion
constant Co is used to determine a corresponding holding time
and/or drawing speed for, respectively, each dot and piecewise
linear portion of G. Thus, in step 2078, the pattern translator
2028 generates the AFM commands for drawing each portion of G and
writes the generated AFM commands to an output file. Note, the
software for generating sequences of AFM commands for drawing
geometric entities is known to those skilled in the art, and such
software is used in, e.g., dot matrix printers. Consequently, in
step 2082, a determination is made as to whether there are
additional geometric entities in the CAD file that need to be
translated into AFM drawing commands. If so, then step 2062 is
again encountered. Alternatively, step 2086 is performed, wherein
the output file of AFM commands is provided as input to the AFM
software drivers 2032.
[0128] Note that further details regarding the pattern translator
2020 are provided in the APPENDIX hereinbelow.
[0129] The invention also provides kits for performing DPN. In one
embodiment, the kit comprises a container holding a patterning
compound and instructions directing that the patterning compound be
used to coat a scanning probe microscope tip and that the coated
tip should be used to apply the patterning compound to the
substrate so as to produce a desired pattern. This kit may further
comprise a container holding a rinsing solvent, a scanning probe
microscope tip a substrate, or combinations thereof. In another
embodiment, the kit comprises a scanning probe microscope tip
coated with a patterning compound. This kit may further comprise a
substrate, one or more containers, each holding a patterning
compound or a rinsing solvent, or both. The substrates, tips,
patterning compounds, and rinsing solvents are those described
above. Any suitable container can be used, such as a vial, tube,
jar, or a well or an array of wells. 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), materials for performing lithography by
methods other than DPN (see the Background section and references
cited therein), and/or materials for wet etching. Finally, the kit
may comprise other reagents and items useful for performing DPN or
any other lithography method, such as reagents, beakers, vials,
etc.
[0130] The invention further provides an AFM adapted for performing
DPN. In one embodiment, this microscope comprises a sample holder
adapted for receiving and holding a substrate and at least one well
holding a patterning compound, the well being positioned so that it
will be adjacent the substrate when the substrate is placed in the
sample holder and and so that it can be addressed by an SPM tip
mounted on the AFM. The sample holder, wells and tips are described
above. In another embodiment, the microscope comprises a plurality
of scanning probe microscope tips and a tilt stage adapted for
receiving and holding a sample holder, the sample holder being
adapted for receiving and holding a substrate. The plurality of
scanning pr e microscope tips and the tilt stage are described
above.
[0131] 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.
[0132] 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.
[0133] 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(CN.sub.2) 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.).
[0134] 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.
[0135] 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.
[0136] 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
[0137] 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).
[0138] 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.
[0139] 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.
[0140] 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.
[0141] 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.
[0142] 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.
[0143] 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.
[0144] 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 with a Variety of Substrates and
"Inks"
[0145] 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.
[0146] 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).
[0147] 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.
[0148] The patterning compounds listed in Table 1 were obtained
from Aldrich Chemical Co. The solvents listed in Table 1 were
obtained from Fisher Scientific.
[0149] 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.
[0150] 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.
[0151] 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 (-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.
[0152] 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.
2TABLE 1 Patterning Compound/ Potential Comments Substrate
Solvent(s) Applications and References Au n-octadecanethiol/ Basic
research Study of inter- acetonitrile, ethanol molecular forces,
Langmuir 10, 3315 (1994) Etching resist for Etchant: KCN/O.sub.2
microfabrication (pH.about.14), J. Vac. Sci. Tech. B, 13, 1139
Dodecanethiol/ Molecular Insulating thin acetonitrile, ethanol
electronics coating on nano- meter scale gold clusters.
Superlattices and Microstructures 18, 275 (1995) n-hexadecanethiol/
Etching resist for Etchant: KCN/O.sub.2 acetonitrile, ethanol
microfabrication (pH.about.14). Langmuir, 15, 300 (1999)
n-docosanethiol/ Etching resist for Etchant: KCN/O.sub.2
acetonitrile, microfabrication (pH.about.14). J. Vac. ethanol Sci.
Technol. B, 13, 2846 (1995) 11-mercapto-1- Surface Capturing
SiO.sub.2 undecanol/ functionalization clusters acetonitrile,
ethanol 16-mercapto-1- Basic research Study of Inter- hexadecanoic
acid/ molecular forces. acetonitrile, ethanol. Langmuir 14, 1508
(1998) Surface Capturing SiO.sub.2, functionalization SnO.sub.2
clusters. J. Am. Chem. Soc., 114, 5221 (1992) Octanedithiol/ Basic
research Study of inter- acetonitrile, ethanol molecular forces.
Jpn. J. Appl. Phys. 37, L299 (1998) Hexanedithiol/ Surface
Capturing gold acetonitrite, ethanol functionalization clusters. J.
Am. Chem. Soc., 114, 5221 (1992) Propariedithiol/ Basic research
Study of inter- acetonitrile, ethanol molecular forces. J. Am.
Chem. Soc., 114, 5221 (1992) .alpha.,.alpha.'-p-xylyldithiol/
Surface Capturing gold acetonitrile, ethanol functionalization
clusters. Science, 272, 1323 (1996) Molecular Conducting nano-
electronics meter scale Science, 272, 1323 (1996)
4,4'-biphenyldithiol/ Surface Capturing gold and acetonitrile,
ethanol functionalization CdS clusters, Inorganica Chemica Acta
242, 115 (1996) Terphenyldithiol/ Surface Capturing gold and
acetonitrile, ethanol functionalization CdS clusters, Inorganica
Chemica Acta 242, 115 (1996) terphenyl- Surface Capturing gold and
diisocyanide/ functionalization CdS clusters, acetonitrile,
Inorganica methylene chloride Chemica Acta 242, 115 (1996)
Molecular Conductive coating electronics on nanometer scale gold
clusters. Superlattices and Microstructures, 18, 275 (1995) DNA/
Gene detection DNA probe to water:acetonitrile detect biological
(1.3) cells. J. Am. Chem. Soc. 119, 8916 (1997) Ag
n-hexadecanethiol/ Etching resist for Etchant: acetonitrile,
ethanol microfabrication Fe(NO.sub.3).sub.3 (pH.about.6).
Microlectron. Eng., 32, 255 (1996) Al 2-mercaptoacetic Surface
Capturing CdS acid/ functionalization clusters. J. Am.
acetonitrile, ethanol Chem. Soc., 114, 5221 (1992) GaAs-100
n-octadecanethiol/ Basic research Self assembled acetonitrile,
ethanol monolayer formation Etching resist for HCl/HNO.sub.3
microfabrication (pH.about.1). 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 hexadecanoic acid/ functionalization CdS clusters acetonitrile,
ethanol octadecyltrichloro- Etching resist for Etchant:HF/NH.sub.4F
silane(OTS, microfabrication (pH.about.2). Appl.
CH.sub.3(ch.sub.2).sub.17SiC- l.sub.3) Phys. Lett., 70, 1.2 nm
thick SAM/ 1593 (1997) hexane APTS, 3-(2- Surface Capturing nano-
Aminoethyl- functionalization meter scale gold amino)propoltri-
clusters Appl. methoxysilane/ Phys. Lett. 70, water 2759 (1997)
Example 3
Atomic Force Microscopy with Coated Tips
[0153] 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.
Piner 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.
[0154] 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.
[0155] 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.
[0156] Two methods for coating an AFM tip with 1-dodecylamine were
explored. The first method involved saturating ethanol or
acetonitiile 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.
[0157] 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.
[0158] 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. 4AB. 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.
[0159] 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. Finer 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.
[0160] 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).
[0161] While the atomic scale imaging ability of the AFM was not
adversely affected by the 1-dodecyl amine 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.
[0162] 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.
[0163] 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.
[0164] 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
[0165] 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. 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. Sc. Technol. B 13, 2846 (1995); and Kim and
M. Lieber, Science 257, 375 (1992).
[0166] 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.
[0167] 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.
[0168] 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.
[0169] 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.
[0170] 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).
[0171] 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).
[0172] 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.
Example 5
Use of DPN to Generate Resists
[0173] Lithographic techniques such as photolithography (Wallraff
and Hinsberg, Chem. Rev., 99:1801(1999)), electron beam lithography
(Wallraff and Hinsberg, Chem. Rev., 99:1801 (1999); Xia et al.,
Chem. Rev., 99:1823 (1999)), and microcontact printing (Xia et al.,
Chem. Rev., 99:1823 (1999)) can be used with varying degrees of
ease, resolution, and cost to generate three-dimensional features
on silicon wafers. DPN is complementary to these other
nanolithographic techniques and can be used with conventional
laboratory instrumentation (an AFM) in routine fashion to generate
patterns of, e.g., alkylthiols on polycrystalline gold substrates,
under ambient conditions. Moreover, DPN offers 15 nm linewidth and
5 nm spatial resolution with conventional AFM cantilevers (see
prior examples; Piner et al., Science, 283:661 (1999); Piner et
al., Langmuir, 15:5457 (1999); Hong et al., Langmuir, 15:7897
(1999); Hong et al., Science, 286:523 (1999)).
[0174] Three-dimensional architectures on and in silicon are vital
to the microelectronics industry and, increasingly, are being
applied to other uses in microfabrication (Xia and Whitesides,
Angew, Chem. Int. Ed. Engl., 37:550 (1998)). For example, the
anisotropic etching of silicon commonly yields narrow grooves,
cantilevers, and thin membranes (Seidel et al., J. Electrochem.
Soc., 137:3612 (1990)), which have been used for sensors of
pressure, actuators, micro-optical components, and masks for
submicron lithography techniques (Seidel, et al., J. Electrochem.
Soc., 137:3612 (1990)). For both the microeletronics applications
and other microfabricated devices, significant advantages are
expected from being able to make smaller feature sizes (Xia and
Whitesides, Angew, Chem. Int. Ed Engl., 37:550 (1998)).
Additionally, the ability to fabricate smaller scale structures can
lead to the discovery or realization of physical and chemical
properties fundamentally different from those typically associated
with larger structures. Examples include Coulomb blockades,
single-electron tunneling, quantum size effects, catalytic
response, and surface plasmon effects (Xia and Whitesides, Angew,
Chem. Int. Ed Engl., 37:550 (1998)). Therefore, a range of
applications is envisioned for the custom-generated solid-state
features potentially attainable through DPN and wet chemical
etching.
[0175] Consequently, the suitability of DPN-generated
nanostructures as resists for generating three-dimensional
multilayered solid-state structures by standard wet etching
techniques was evaluated in a systematic study, the results of
which are reported in this example. In this study, DPN was used to
deposit alkylthiol monolayer resists on Au/Ti/Si substrates.
Subsequent wet chemical etching yielded the targeted
three-dimensional structures. Many spatially separated patterns of
the monolayer resists can be deposited by DPN on a single AU/Ti/Si
chip and, thus, the effects of etching conditions can be examined
on multiple features in combinatorial fashion.
[0176] As diagrammed in FIG. 12, in a typical experiment in this
study, DPN was used to deposit alkylthiols onto an Au/Ti/Si
substrate. It has been well established that alkyithiols form
well-ordered mono layers on Au thin films that protect the
underlying Au from dissolution during certain wet chemical etching
procedures (Xia et al., Chem. Mater., 7:23 32 (1995); Kumar et al.,
J. Am. Chem. Soc., 114:9188 (1992)), and this appears to also hold
true for DPN-generated resists (see below). Thus, the Au, Ti, and
SiO.sub.2 which were not protected by the monolayer could be
removed by chemical etchants in a staged procedure (FIG. 12, panels
b-e). This procedure yielded "first-stage" three-dimensional
features: multilayer, Au-topped features on the Si substrate (FIG.
12, panel b). Furthermore, "second-stage" features were prepared by
using the remaining Au as an etching resist to allow for selective
etching of the exposed Si substrate (FIG. 12, panels c and d).
Finally, the residual Au was removed to yield final-stage all-Si
features, FIG. 12, panel e. Thus, DPN can be combined with wet
chemical etching to yield three-dimensional features on Si(100)
wafers with at least one dimension on the sub-100 nm length
scale.
[0177] Specifically, FIG. 12 diagrams the procedure used to prepare
nanoscale features on Si wafers. First, polished single-crystalline
Si(100) wafers were coated with 5 nm of Ti, followed by 10 nm of Au
by thermal evaporation. The Si(100) wafers (4" diameter (1-0-0)
wafers; 3-4.9 ohm/cm resistivity; 500-550 .mu.m thickness) were
purchased from Silicon Quest International, Inc. (Santa Clara,
Calif.). Thermal evaporation of 5 nm of Ti (99.99%; Alfa Aesar;
Ward Hill, Mass.) followed by 10 nm of Au (99.99%; D.F. Goldsmith;
Evanston, Ill.) was accomplished using an Edwards Auto 306 Turbo
Evaporator equipped with a turbopump (Model EXT510) and an Edwards
FTM6 quartz crystal microbalance to determine film thickness. Au
and Ti depositions were conducted at room temperature at a rate of
1 nm/second and a base pressure of <9.times.10.sup.-7 mb.
[0178] After Au evaporation, the following procedure was performed
on the substrates: a) DPN was used to deposit patterns of ODT, b)
Au and Ti were etched from the regions not protected by the ODT
monolayers using a previously reported ferri/ferrocyanide based
etchant (Xia et al., Chem. Mater., 7:2332 (1995)), c) residual Ti
and SiO.sub.2 were removed by immersing the sample into a 1% HF
solution (note: this procedure also passivates the exposed Si
surfaces with respect to native oxide growth) (Ohmi, J.
Electrochem. Soc., 143:2957(1996)), and d) the remaining Si was
etched anisotropically by minor modifications of a previously
reported basic etchant (Seidel et al., J. Electrochem. Soc.,
137:3612 (1990)). The topography of the resulting wafers was
evaluated by AFM and SEM.
[0179] All DPN and all AFM imaging experiments were carried out
with a Thermomicroscopes CP AFM and conventional cantilevers
(Thermomicroscopes sharpened Microlever A, force constant 0.05 N/m,
Si.sub.3N.sub.4). A contact force of 0.5 nN was typically used for
DPN patterning. To minimize piezo tube drift problems, a 100-.mu.m
scanner with closed loop scan control was used for all of the
experiments. For DPN, the tips were treated with ODT in the
following fashion: 1) tips were soaked in 30% H.sub.2O, H,SO.sub.4
(3:7) (caution: this mixture reacts violently with organic
material) for 30 minutes, 2) tips were rinsed with water, 3) tips
were heated in an enclosed canister (approximately 15 cm.sup.3
internal volume) with 200 mg ODT at 60.degree. C. for 30 minutes,
and 4) tips were blown dry with compressed difluoroethane prior to
use. Typical ambient imaging conditions were 30% humidity and
23.degree. C., unless reported otherwise. Scanning electron
microscopy (SEM) was performed using a Hitachi SEM equipped with
EDS detector.
[0180] A standard ferri/ferrocyanide etchant was prepared as
previously reported (Xia et al., Chem. Mater., 7:2332 (1995)) with
minor modification: 0.1 MNa.sub.1SO.sub.3, 1.0 M KOH, 0.01 M
K.sub.3Fe(CN).sub.5, 0.001 M K.sub.4Fe(CN).sub.6 in nanopure water.
Au etching was accomplished by immersing the wafer in this solution
for 2-5 minutes while stirring. The HF etchant (1% (v:v) solution
in nanopure water) was prepared from 49% HF and substrates were
agitated in this solution for 10 seconds. Silicon etching was
accomplished by immersing the wafer in 4 M KOH in 15% (v:v)
isopropanol in nanopure water at 55.degree. C. for 10 seconds while
stirring (Seidel et al., J. Electrochem. Soc., 137:3612 (1990)).
Final passivation of the Si substrate with respect to SiO, growth
was achieved by immersing the samples in 1% HF for 10 seconds with
mild agitation. Substrates were rinsed with nanopure water after
each etching procedure. To remove residual Au, the substrates were
cleaned in O.sub.2 plasma for 3 minutes and soaked in aqua regia
(3:1 HCl:HNO.sub.3) for 1 minute, followed by immersing the samples
in 1% HF for 10 seconds with mild agitation.
[0181] FIG. 13A shows the AFM topography images of an AU/Ti/Si chip
patterned according to the procedure outlined in FIG. 12, panels
a-d. This image shows four pillars with a height of 55 nm formed by
etching an AU/Ti/Si chip patterned with four equal-sized dots of
ODT with center-to-center distances of 0.8 .mu.m. Each ODT dot was
deposited by holding the AFM tip in contact with the Au surface for
2 seconds. Although the sizes of the ODT dots were not measured
prior to etching, their estimated diameters were approximately
100nm. This estimate is based upon the measured sizes of ODT "test"
patterns deposited with the same tip on the same surface
immediately prior to deposition of the ODT dots corresponding to
the shown pillars. The average diameter of the shown pillar tops
was 90 nm with average base diameter of 240 nm. FIG. 13B shows a
pillar(55 nm height, 45 nm top diameter, and 155 nm base diameter)
from a similarly patterned and etched region on the same Au/Ti/Si
substrate. The cross-sectional topography trace across the pillar
diameter showed a flat top and symmetric sidewalls, FIG. 13C. The
shape of the structure may be convoluted by the shape of the AFM
tip (approximately 10 nm radius of curvature), resulting in side
widths as measured by AFM which may be larger than the actual
widths.
[0182] Additionally, an AU/Ti/Si substrate was patterned with three
ODT lines drawn by DPN (0.4 .mu.m/second, estimated width of each
ODT line is 100 nm) with 1 .mu.m center-to-center distances. FIG.
14A shows the AFM topography image after etching this substrate
according to FIG. 12, panels a-d. The top and base widths are 65 nm
and 415 nm, respectively, and line heights are 55 nm. FIG. 14B
shows a line from a similarly patterned and etched region on the
same Au/Ti/Si wafer, with a 50 nm top width, 155 nm base width, and
55 nm height. The cross-sectional topography trace across the line
diameter shows a flat top and symmetric sidewalls (FIG. 14C).
[0183] FIGS. 15 and 16 show the feature-size variation possible
with this technique. In FIG. 15A, the ODT-coated AFM tip was held
in contact with the surface for varying lengths of time (16-0.062
seconds) to generate various sized dots with 2 .mu.m
center-to-center distances which subsequently yielded etched
three-dimensional structures with top diameters ranging from 1.47
.mu.m to 147 nm and heights of 80 nm. The top diameters as measured
by SEM differed by less than 15% from the diameters measured from
the AFM images, compare FIGS. 15A and 15B. Additionally, energy
dispersive spectroscopy (EDS) showed the presence of Au on the
pillar tops whereas Au was not observed in the areas surrounding
the elevated micro- and nanostructures. As expected, the diameters
of the micro- and nano-trilayer structures correlated with the size
of the DPN-generated resist features, which was directly related to
tip-substrate contact time, FIG. 15C. Line structures were also
fabricated in combinatorial fashion, FIG. 16. ODT lines were drawn
at a scan rate varying from 0.2-2.8 .mu.m/second with 1 .mu.m
center-to-center distances. After etching, these resists afforded
trilayer structures, all with a height of 80 nm and top line widths
ranging from 505 to 50 nm, FIG. 16. The field emission scanning
electron micrograph of the patterned area looks comparable to the
AFM image of the same area with the top widths as determined by the
two techniques being within 15% of one another, compare FIGS. 16A
and 16B.
[0184] In conclusion, it has been demonstrated that DPN can be used
to deposit monolayer-based resists with micron to sub-100 nm
dimensions on the surfaces of Au/Ti/Si trilayer substrates. These
resists can be used with wet chemical etchants to remove the
unprotected substrate layers, resulting in three-dimensional
solid-state feature with comparable dimensions. It is important to
note that this example does not address the ultimate resolution of
solid-state nano structure fabrication by means of DPN. Indeed, it
is believed that the feature size will decrease through the use of
new "inks" and sharper "pens." Finally, this work demonstrates the
potential of using DPN to replace the complicated and more
expensive hard lithography techniques (e.g. e-beam lithography) for
a variety of solid-state nanolithography applications.
Example 6
Multi-Pen Nanoplotter for Serial and Parallel DPN
[0185] The largest limitation in using scanning probe methodologies
for doing ultra-high-resolution nanolithography over large areas
derives from the serial nature of most of these techniques. For
this reason, scanning probe lithography (SPL) methods have been
primarily used as customization tools for preparing and studying
academic curiosities (Snow et al., Appl. Phys. Lett., 75:1476
(1999); Luthi et al., Appl. Phys. Lett., 75:1314 (1999); Bottomley,
Anal. Chem., 70:425R (1998); Schoer and Crooks, Langmuir, 13:2323
(1997); Xu and Liu, Langmuir, 13:127 (1997); Nyffenegger and
Penner, Chem. Rev., 97:1195 (1997); Sugimur and Nakagiri, J. Vac.
Sci. Technol. A, 14:1223 (1996); Muller et al., J. Vac. Sci.
Technol. B, 13:2846 (1995); Jaschke and Butt, Langmuir, 11:1061
(1995); Kim and Lieber, Science, 257:375 (1992)). If SPL
methodologies are ever to compete with optical or even stamping
lithographic methods for patterning large areas (Xia et al., Chem.
Rev., 99:1823 (1999); Jackman et al., Science, 269:664(1995); Chou
et al., Appl. Phys. Lett., 67:3114(1995)), they must be converted
from serial to parallel processes. Several important steps have
been taken in this direction. For example, researchers have
developed a variety of different scanning multiple probe
instruments (Lutwyche et al., Sens. Actuators A, 73:89 (1999);
Vettiger et al, Microelectron Eng., 46:11 (1999); Mime et al.,
Appl. Phys. Lett., 73:1742 (1998); Tsukamoto et al., Rev. Sd.
Instrum., 62:1767 (1991)), and some have begun to use these
instruments for parallel SPL. In particular, Quate and coworkers
have shown that as many as 50 tips could be used at once (Minne et
al., Appl. Phys. Lett., 73:1742 (1998)), and with such a strategy,
both imaging and patterning speeds could be dramatically improved.
However, a major limitation of all parallel SPL methods thus far
developed is that each tip within the array needs a separate
feedback system, which dramatically increases the instrumentation
complexity and cost. One of the reasons separate feedback systems
are required in such a process is that tip-substrate contact force
influences the line width and quality of the patterned structure.
Although parallel scanning tunneling microscope (STM) lithography
has not yet been demonstrated, such a process would presumably
require a feedback system for each tip that allows one to maintain
constant tunneling currents. Like most other SPL methods, DPN thus
far has been used exclusively in a serial format. Herein, a method
for doing parallel or single pen soft nanolithography using an
array of cantilevers and a conventional AFM with a single feed back
system is reported.
[0186] There is a key scientific observation that allows one to
transform DPN from a serial to parallel process without
substantially complicating the instrumentation required to do DPN.
It has been discovered that features (e.g. dots and lines)
generated from inks such as 1-octadecanethiol (ODT), under
different contact forces that span a two-order of magnitude range,
are virtually identical with respect to diameter and line-width,
respectively. Surprisingly, even patterning experiments conducted
with a small negative contact force, where the AFM tip bends down
to the surface, exhibit ink transport rates that are comparable to
experiments executed with the tip-substrate contact force as large
as 4 nN (FIG. 19). These experiments clearly showed that, in DPN
writing, the ink molecules migrate from the tip through the
meniscus to the substrate by diffusion, and the tip is simply
directing molecular flow.
[0187] The development of an eight pen nanoplotter capable of doing
parallel DPN is described in this example. Significantly, since DPN
line width and writing speed are independent of contact force, this
has been accomplished in a configuration that uses a single tip
feedback system to monitor a tip with dual imaging and writing
capabilities (designated the "imaging tip"). In parallel writing
mode, all other tips reproduce what occurs at the imaging tip in
passive fashion. Experiments that demonstrate eight-pen parallel
writing, ink and rinsing wells, and "molecular corralling" by means
of a nanoplotter-generated structure are reported.
[0188] All experiments were performed on a Thermomicroscopes MS AFM
equipped with a closed loop scanner that minimizes thermal drift.
Custom DPN software (described above) was used to drive the
instrument. The instrument has a 200 mm.times.200 mm sample holder
and an automated translation stage.
[0189] The intention in transforming DPN into a parallel process
was to create an SPL method that allows one to generate multiple
single-ink patterns in parallel or a single multiple-ink pattern in
series. This tool would be the nanotechnologist's equivalent of a
multiple-pen nanoplotter with parallel writing capabilities. To
accomplish this goal, several modifications of the AFM and DPN
process were required (see FIGS. 17 and 18).
[0190] First, a tilt stage (purchased from Newport Corporation) was
mounted on the translation stage of the AFM. The substrate to be
patterned was placed in the sample holder, which was mounted on the
tilt stage. This arrangement allows one to control the orientation
of the substrate with respect to the ink coated tips which, in
turn, allows one to selectively engage single or multiple tips
during a patterning experiment (FIG. 17).
[0191] Second, ink wells, which allow one to individually address
and ink the pens in the nanoplotter, were fabricated. Specifically,
it has been found that rectangular pieces of filter paper soaked
with different inks or solvents can be used as ink wells and
rinsing wells, respectively (FIG. 17). The filter-paper ink and
rinsing wells were located on the translation stage proximate the
substrate. An AFM tip can be coated with a molecular ink of
interest or rinsed with a solvent simply by making contact with the
appropriate filter-paper ink or rinsing well for 30 seconds
(contact force=1 nN).
[0192] Finally, a multiple tip array was fabricated simply by
physically separating an array of cantilevers from a commercially
available wafer block containing 250 individual cantilevers
(Thermomicroscopes Sharpened Microlevers C, force constant=0.01
N/m), and then, using that array as a single cantilever (FIG. 18).
The array was affixed to a ceramic tip carrier that comes with the
commercially acquired mounted cantilevers and was mounted onto the
AFM tip holder with epoxy glue (FIG. 18).
[0193] For the sake of simplicity, experiments involving only two
cantilevers in the array will be described first. In parallel
writing, one tip, designated "the imaging tip," is used for both
imaging and writing, while the second tip is used simply for
writing. The imaging tip is used the way a normal AFM tip is used
and is interfaced with force sensors providing feedback; the
writing tips do not need feedback systems. In a patterning
experiment, the imaging tip is used to determine overall surface
topology, locate alignment marks generated by DPN, and
lithographically pattern molecules in an area with coordinates
defined with respect to the alignment marks (Example 4 and Hong et
al., Science, 286:523 (1999)). With this strategy, the writing
tip(s) reproduce the structure generated with the imaging tip at a
distance determined by the spacing of the tips in the cantilever
array (600 .mu.m in the case of a two pen experiment).
[0194] In a typical parallel, multiple-pen experiment involving a
cantilever array, each tip was coated with an ink by dipping it
into the appropriate ink well. This was accomplished by moving the
translation stage to position the desired ink well below the tip to
be coated and lowering the tip until it touched the filter paper.
Contact was maintained for 30 seconds, contact force=1 nN. To begin
parallel patterning, the tilt stage was adjusted so that the
writing tip was 0.4 .mu.m closer to the sample than the imaging
tip. The tip-to-sample distances in an array experiment can be
monitored with the Z-stepper motor counter. The laser was placed on
the imaging tip so that during patterning both tips were in contact
with the surface (FIG. 17).
[0195] The first demonstration of parallel writing involved two
tips coated with the same ink, ODT (FIG. 20A). In this experiment,
two one-molecule-thick nanostructures comprised of ODT were
patterned onto a gold surface by moving the imaging tip along the
surface in the form of a square (contact force.about.0.1 nN;
relative humidity 30%; writing speed=0.6 .mu.m/sec). Note that the
line-widths are nearly identical and the nanostructure registration
(orientation of the first square with respect to the second) is
near-perfect.
[0196] Parallel patterning can be accomplished with more than one
ink. In this case the imaging tip was placed in a rinsing well to
remove the ODT ink and then coated with 16-mercaptohexadecanoic
acid (MBA) by immersing it in an MBA ink well. The parallel
multiple-ink experiment was then carried out in a manner analogous
to the parallel single ink experiment under virtually identical
conditions. The two resulting nanostructures can be differentiated
based upon lateral force but, again, are perfectly aligned due to
the rigid, fixed nature of the two tips (FIG. 20B). Interestingly,
the line-widths of the two patterns were identical. This likely is
a coincidental result since feature size and line width in a DPN
experiment often depend on the transport properties of the specific
inks and ink loading.
[0197] A remarkable feature of this type of nanoplotter is that, in
addition to offering parallel writing capabilities, one can operate
the system in serial fashion to generate customized nanostructures
made of different inks. To demonstrate this capability, a
cantilever array that had a tip coated with ODT and a tip coated
with MHA was utilized. The laser was focused on the ODT coated tip,
and the tilt stage was adjusted so that only this tip was in
contact with the surface (FIG. 17). The ODT coated tip was then
used to generate the vertical sides of a cross on a Au surface
(contact force.about.0.1 nN; relative humidity.about.30%; writing
speed=1.3 .mu.m/second) (FIG. 21 A). The laser was then moved to
the MBA coated tip, and the tilt stage was readjusted so that only
this tip was in contact with surface. The MHA tip was then used to
draw the 30 nm wide horizontal sides of the nanostructure ("nano"
refers to line width) (FIG. 21A). Microscopic ODT alignment marks
deposited on the periphery of the area to be patterned were used to
locate the initial nanostructure as described above (see also
Example 4 and Hong et al., Science, 286:523 (1999)).
[0198] This type of multiple ink nanostructure with a bare gold
interior would be impossible to prepare by stamping methodologies
or conventional nanolithography methods, but was prepared in five
minutes with the multiple-pen nanoplotter. Moreover, this tool and
these types of structures can now be used to begin evaluating
important-issues involving molecular diffusion on the nanometer
length scale and across nanometer wide molecule-based barriers. As
a proof-of-concept, the diffusion of MHA from a tip to the surface
within this type of "molecule-based corral" was examined. As a
first step, a cross shape was generated with a single ink, ODT
(contact force.about.0.1 nN; relative humidity.about.30%; writing
speed=0.5 .mu.m/second). Then, an MHA coated tip was held in
contact with the surface for ten minutes at the center of the cross
so that MHA molecules were transported onto the surface and could
diffuse out from the point of contact. Importantly, even 80 nm wide
ODT lines acted as a diffusion barrier, and MBA molecules were
trapped inside the ODT cross pattern (FIG. 21B). When the
horizontal sides of the molecular corral are comprised of MHA
barriers, the MHA molecules diffuse from tip onto the surface and
over the hydrophilic MHA barriers. Interestingly, in this two
component nanostructure, the MBA does not go over the MHA barriers,
resulting in an anisotropic pattern (FIG. 21C). Although it is not
known yet if the corral is changing the shape of the meniscus,
which in turn controls ink diffusion, or alternatively, the ink is
deposited and then migrates from the point of contact to generate
this structure, this type of proof-of-concept experiment shows how
one can begin to discover and study important interfacial processes
using this new nanotechnology tool.
[0199] The parallel nanoplotting strategy reported herein is not
limited to two tips. Indeed, it has been shown that a cantilever
array consisting of eight tips can be used to generate
nanostructures in parallel fashion. In this case, each of the eight
tips was coated with ODT. The outermost tip was designated as the
imaging tip and the feedback laser was focused on it during the
writing experiment. To demonstrate this concept, four separate
nanostructures, a 180 nm dot (contact force.about.0.1 nN, relative
humidity=26%, contact time=1 second), a 40 nm wide line, a square
and an octagon (contact force 0.1 nN, relative humidity =26%,
writing speed=0.5 .mu.m/second) were generated and reproduced in
parallel fashion with the seven passively following tips (FIG. 22).
Note that there is a less than 10% standard deviation in line width
for the original nanostructures and the seven copies.
[0200] In summary, DPN has been transformed from a serial to a
parallel process and, through such work, the concept of a
multiple-pen nanoplotter with both serial and parallel writing
capabilities has been demonstrated. It is important to note that
the number of pens that can be used in a parallel DPN experiment to
passively reproduce nanostructures is not limited to eight. Indeed,
there is no reason why the number of pens cannot be increased to
hundreds or even a thousand pens without the need for additional
feedback systems. Finally, this work will allow researchers in the
biological, chemical, physics, and engineering communities to begin
using DPN and conventional AFM instrumentation to do automated,
large scale, moderately fast, high-resolution and alignment
patterning of nanostructures for both fundamental science and
technological applications.
Example 7
Use of DPN to Prepare Combinatorial Arrays
[0201] A general method for organizing micro- and nanoparticles on
a substrate could facilitate the, formation and study of photonic
band gap materials, make it possible to generate particle arrays
for analysis of the relationship between pattern structure and
catalytic activity, and enable formation of single protein particle
arrays for proteomics research. While several methods have been
reported for assembling collections of particles onto patterned
surfaces (van Blaaderen et al., Nature 385:321-323 (1997); Sastry
et al., Langmuir 16:3553-3556 (2000); Tien et al., Langmuir
13:5349-5355 (1997); Chen et al., Langmuir 16:7825-7834(2000);
Vossmeyer et al., J. Appl. Phys. 84:3664-3670(1998); Qin et al.,
Adv. Mater. 11:1433-1437 (1999)), a major challenge lies in the
selective immobilization of single particles into pre-determined
positions with respect to adjacent particles.
[0202] A strategy for chemically and physically immobilizing a wide
variety of particle types and sizes with a high degree of control
over particle placement calls for a soft lithography technique
capable of high-resolution patterning, but also with the ability to
form patterns of one or more molecules with precision alignment
registration. DPN is such a tool. This example demonstrates
combinatorial arrays produced by DPN, focusing on the problem of
particle assembly in the context of colloidal crystallization.
[0203] Recently, conventional sedimentation methods for preparing
colloidal crystals consisting of close-packed layers of polymer or
inorganic particles (Park et al., Adv. Mater. 10:1028-1032 (1998),
and references cited therein; Jiang et al., Chem. Mater.
11:2132-2140 (1999)) have been combined with polymer templates,
fabricated by e-beam lithography, to form high quality
single-component structures (van Blaaderen et al., Nature
385:321-323 (1997)). However, sedimentation or solvent evaporation
routes do not offer the element of chemical control over particle
placement. Herein, a DPN-based strategy for generating charged
chemical templates to study the assembly of single particles into
two-dimensional square lattices is described.
[0204] The general method (outlined in FIG. 23) is to form a
pattern on a substrate composed of an array of dots of an ink which
will attract and bind a specific type of particle. For the present
studies, MHA was used to make templates on a gold substrate, and
positively-charged protonated amine- or amidine-modified
polystyrene spheres were used as particle building blocks.
[0205] Gold coated substrates were prepared as described in Example
5. For in situ imaging experiments requiring transparent
substrates, glass coverslips (Corning No. 1 thickness, VWR,
Chicago, Ill.) were cleaned with Ar/O--, plasma for 1 minute, then
coated with 2 nm of Ti and 15 nm of Au. The unpattemed regions of
the gold substrate were passivated by immersing the substrate in a
1 mM ethanolic solution of another alkanethiol, such as ODT or
cystamine. Minimal, if any, exchange took place between the
immobilized MHA molecules and the ODT or cystamine in solution
during this treatment, as evidenced by lateral force microscopy of
the substrate before and after treatment with ODT.
[0206] The gold substrates were patterned with MHA to form arrays
of dots. DPN patterning was carried out under ambient laboratory
conditions (30% humidity, 23.degree. C.) as described in Example 5.
It is important to note that the carboxylic acid groups in the MHA
patterns were deprotonated providing an electrostatic driving force
for particle assembly. (Vezenov et al., J. Am. Chem. Soc.
119:2006-2015 (1997))
[0207] Suspensions of charged polystyrene latex particles in water
were purchased from either Bangs Laboratories (0.93 .mu.m, Fishers,
Ind.) or IDC Latex (1.0 .mu.m and 190 nm, Portland, Oreg.).
Particles were rinsed free of surfactant by centrifugation and
redispersion twice in distilled deionized water (18.1 M.OMEGA.)
purified with a Barnstead (Dubuque, Iowa) NANOpure water system.
Particle assembly on the substrate was accomplished by placing a 20
.mu.1 droplet of dispersed particles (10% wt/vol in deionized
water) on the horizontal substrate in a humidity chamber (100%
relative humidity). Gentle rinsing with deionized water completed
the process.
[0208] Optical microscopy was performed using the Park Scientific
CP AFM optics (Thermomicroscopes, Sunnyvale, Calif.) or, for in
situ imaging, an inverted optical microscope (Axiovert 100A, Carl
Zeiss, Jena, Germany) operated in differential interference
contrast mode (DIC). Images were captured with a Penguin 600 CL
digital camera (Pixera, Los Gatos, Calif.). Intermittent-contact
imaging of particles was performed with a Thermomicroscopes MS AFM
using silicon ultralevers (Thermomicroscopes, spring constant=3.2
N/m). Lateral force imaging was carried out under ambient
laboratory conditions (30% humidity, 23.degree. C.) and as
previously reported (Weinberger et al., Adv. Mater. 12:1600-1603
(2000)).
[0209] In a typical experiment involving 0.93 .mu.m diameter
particles, multiple templates were monitored simultaneously for
particle assembly by optical microscopy. In these experiments, the
template dot diameter was varied to search for optimal conditions
for particle-template recognition, FIG. 24 (left to right). After 1
hour of particle assembly, the substrates were rinsed with
deionized water, dried under ambient laboratory conditions, and
then imaged by optical microscopy, FIG. 25. The combinatorial
experiment revealed that the optimum size of the template pad with
which to immobilize a single particle of this type in high registry
with the pattern was approximately 500-750 nm. It is important to
note that drying of the substrate tended to displace the particles
from their preferred positions on the template, an effect that has
been noted by others with larger scale experiments (Aizenberg et
al., Phys. Rev. Lett. 84:2997-3000 (2000)). Indeed, evidence for
better, in fact near-perfect, particle organization is obtained by
in situ imaging of the surface after 1 .mu.m amine-modified
particles have reacted with the template for 1 hour, FIG. 26.
[0210] Single particle spatial organization of particles on the
micron length-scale has been achieved by physical means, for
instance using optical tweezers (Mio et a]., Langmuir 15:8565-8568
(1999)) or by sedimentation onto e-beam lithographically patterned
polymer films (van Blaaderen et al., Nature 385:321-323 (1997)).
However, the DPN-based method described here offers an advantage
over previous methods because it provides flexibility of length
scale and pattern type, as well as a means to achieve more robust
particle array structures. For instance, DPN has been used to
construct chemical templates which can be utilized to prepare
square arrays of 190 nm diameter amidine-modified polystyrene
particles. Screening of the dried particle arrays using non-contact
AFM or SEM imaging revealed that 300 nm template dots of MHA,
spaced 570 nm apart, with a surrounding repulsive monolayer of
cystamine, were suitable for immobilizing single particles at each
site in the array, FIG. 27A. However, MHA dots of diameter and
spacing of 700 nm and 850 nm resulted in immobilization of multiple
particles at some sites, FIG. 27B.
[0211] Similar particle assembly experiments conducted at pH<5
or >9 resulted in random, non-selective particle adsorption,
presumably due to protonation of the surface acid groups or
deprotonation of particle amine or amidine groups. These
experiments strongly suggested that the particle assembly process
was induced by electrostatic interactions between charged particles
and patterned regions of the substrate.
[0212] In conclusion, it has been demonstrated that DPN can be used
as a tool for generating combinatorial chemical templates with
which to position single particles in two-dimensional arrays. The
specific example of charged alkanethiols and latex particles
described here will provide a general approach for creating
two-dimensional templates for positioning subsequent particle
layers in predefined crystalline structures that may be composed of
single or multiple particle sizes and compositions. In a more
general sense, the combinatorial DPN method will allow researchers
to efficiently and quickly form patterned substrates with which to
study particle-particle and particle-substrate interactions,
whether the particles are the dielectric spheres which comprise
certain photonic band-gap materials, metal, semiconductor particles
with potential catalytic or electronic properties, or even living
biological cells and macrobiomolecules.
APPENDIX
[0213] The program is written in the Microsoft Visual Basic.
[0214] This Form_DPNWrite is a core subroutine of the pattern
interpreter.
[0215] The processes which should be done before the execution of
the subroutine are:
[0216] 1) Users should design patterns utilizing the user-interface
subroutine.
[0217] 2) The patterns designed by the users should be converted
into series of dots and lines via well-known subroutines. The dots
and lines should be saved in the variables, MyDot(i) and MyLine(i),
respectively.
[0218] 3) The diffusion constant C should be measured or retrieved
from the table for the current tip, substrate, substance and
environmental conditions, and it should be saved in the variable,
Diffusion.
[0219] The major functions of this subroutine are:
[0220] 1) Calculate the holding time and speed for the basic
patterns, dots and lines, respectively.
[0221] 2) Save the corresponding command lines in the script
file.
[0222] 3) Ask the SPM software to run the script file to perform
DPN writing. MyDot(i) is an array of DPNDot objects (class).
Several important properties of the DPNDot object are X Y, Size,
HoldTime. MyDot(i) represents a dot pattern with a radius of
MyDot(i).Size at the position of (MyDot(i)X, MyDot(i). Y).
MyLine(i) is an array of DPNLine objects (class). Several important
properties of the DPNLine objects are X1, Y1, X2, Y2, DPNWidth,
Repeat, Speed. MyLine(i) represents a line pattern connecting
between (X1, Y1) and (X2, Y2) with a linewidth of DPNWidth. Repeat
is an optional parameter and its default value is 1. By specifying
Repeat, users can specify whether the line will be drawn by one or
multiple sweeps of the SPM tip.
[0223] Program starts here:
[0224] Public Sub Form DPNWrite( )
[0225] `Calculate the holding time for each dot and save it in
MyDot(i).Hold Time.`
[0226] For i=1 To MyDotNum
[0227]
MyDot(i).HoldTime=Round(3.14159*MyDot(i).Size*MyDot(i).Size/Diffusi-
on, 5)
[0228] Next i
[0229] `Calculate the speed for each line and save it in
MyLine(i).Speed.`
[0230] For i=1 To MyLineNum
[0231]
MyLine(i).Speed=Round(Diffusion*MyLine(i).Repeat/MyLine(i).DPN-Widt-
h, 5)
[0232] Next i
[0233] `Create the script file which will store all the command
lines which can be recognized by SPM software`
[0234] Open "c:.backslash.dpnwriting.backslash.nanoplot.scr" For
Output As #1
[0235] `In the following lines, Command 1.about.10 represent
command lines specific for each commercial system for the drawing
system 2030, and accordingly are dependent upon, e.g, the atomic
force microscope system utilized as the drawing system.`
[0236] `Add the command for the SPM system initialization to the
script file.`
[0237] Print #1, "Command 1: Set up the Drawing System."
[0238] Print #1, "Command 2: Separate the tip from the
substrate."
[0239] `Add the commands for dot patterns to the script file.`
[0240] For i=1 To MyDotNum
[0241] If MyDot(i).HoldTime>0 Then
[0242] Print #1, "Command 3: Move the tip to the position of the
dot."
[0243] Print #1, "Command 4: Approach the tip to make a contact
with the substrate."
[0244] Print #1, "Command 5: Hold the tip for the period of
[0245] MyDot(i). HoldTime."
[0246] End If
[0247] Print #1, "Command 6: Separate the tip from the
substrate."
[0248] Next i
[0249] `Add the commands for line patterns to the script file.`
[0250] For i=1 To MyLineNum
[0251] If MyLine(i).Speed>0 Then
[0252] Print #1, "Command 7: Move the tip to the initial position,
(X1, Y1)"
[0253] Print #1, "Command 8: Approach the tip to make a contact
with the substrate."
[0254] Print #1, "Command 9: Sweep the tip to (X2, Y2) with
MyLine(:).Speed."
[0255] End If
[0256] Print #1, "Command 10: Separate the tip from the
substrate."
[0257] Next i
[0258] Close #1
[0259] `Have the drawing system 2030 execute the commands in the
script file.` `The method to have the AFM software drivers 2032 run
the script file depends on the commercial drawing system 2030 used.
The following is one example where Shell Visual Basic function is
utilized.`
[0260]
DoDPN=Shell("c:.backslash.spmsoftware.backslash.spmsoftware.exe-x
c:.backslash.dpnwriting.backslash.-nanoplot.scr",
vbMinimizedFocus)
[0261] End Sub
* * * * *