Nanolithography and Nanochemistry: Probe ... - Multiple Choices

Reviews
U. S. Schubert and D. Wouters
Nanolithography
Nanolithography and Nanochemistry: Probe-Related
Patterning Techniques and Chemical Modification for
Nanometer-Sized Devices
Daan Wouters and Ulrich S. Schubert*
Keywords:
atomic force microscopy · lithography ·
nanostructures · oxidation ·
scanning tunneling microscopy
Dedicated to Professor Dieter Schubert
on the occasion of his 65th birthday
Angewandte
Chemie
2480 2004 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim DOI: 10.1002/anie.200300609 Angew. Chem. Int. Ed. 2004, 43, 2480 – 2495

Nanolithography
Angewandte
Chemie
The size regime for devices produced by photolithographic techniques
is limited. Therefore, other patterning techniques have been intensively
studied to create smaller structures. Scanning-probe-based patterning
techniques, such as dip-pen lithography, local force-induced
patterning, and local-probe oxidation-based techniques are highly
promising because of their relative ease and widespread availability.
The latter of these is especially interesting because of the possibility of
producing nanopatterns for a broad range of chemical and physical
modification and functionalization processes; both the production of
nanometer-sized electronic devices and the formation of devices
involving (bio)molecular recognition and sensor applications is
possible. This Review highlights the development of various scanning
probe systems and the possibilities of local oxidation methods, as well
as giving an overview of state-of-the-art nanometer-sized devices, and
a view of future development.
1. Introduction
Nanotechnology is currently at the center of attention in
both the media and in public opinion as well as in
governmental and industrial research programs. Large funding
programs related to nanotechnology were recently
initiated in Europe, Japan, and the USA. [1,2] This development
is reflected by a shift of attention in academic and
industrial research centers. The envisioned fields and their
potential technological impact on society range from “smart”
drugs or sensitive DNA/protein sensors to nanostructured
electronic devices with applications in information storage,
optical computers, solar cells, and energy storage, as well as
novel materials with improved properties such as “intelligent”
and self-cleaning or color-changing coatings. [3–6] It should be
noted that many of these envisioned improved properties
relate to changes and/or control of surface properties. This
directly implies the need for well-defined, controllable,
reliable, and affordable surface modification and characterization
techniques. In general, two approaches for the
creation of nanostructured surfaces are available. The first,
the so-called “top-down” approach, epitomizes efforts by
physicists and engineers to reduce the size of structures with
techniques that are currently available. Most results using this
strategy originate from the downscaling of lithographic
techniques in semiconductor research. The second “bottomup”
approach has been developed by (bio)chemists, who have
considered the self-assembling processes in nature when
creating similar patterns that utilize noncovalent molecular
interactions such as hydrogen bonding, p–p stacking, and/or
metal–ligand coordination. [7–11] Although the bottom-up
approach is clearly interesting, and can lead to unique and
interesting structures, it also has some clear disadvantages:
The ability of patterning alone is insufficient for successful
application in advanced (nano)devices, since besides size and
structure, control over both direction and position is also
essential. Moreover, the ability to characterize the created
From the Contents
1. Introduction 2481
2. Physical Probe Lithography
Techniques 2483
3. Oxidative Probe Lithography
Techniques 2486
4. Conclusions and Outlook 2493
structures and the feasibility of producing
these structures on large areas
is of central interest.
As a top-down approach, lithography
represents an important surfacepatterning
technique. Lithography was
introduced in 1798 as a printing tool by
Alois Senefelder, who found that ink adsorbed to an image
drawn with a greasy fluid onto limestone (lithographic stones,
Figure 1) could be used to transfer the image to a piece of
Figure 1. Lithographic stones were introduced by Alois Senefelder. [12]
paper pressed against the stone. [12–15] Since then, this printing
technique has evolved dramatically and is nowadays not only
used for printing but also for the (chemical) modification of
substrates on a submicrometer scale, and is often referred to
as microcontact printing. [16–18] The most prominent application
of lithography is that of UV-mask lithography, which is
widely used in the semiconductor industry. With the development
of (computer) chips and their increasing complexity, the
requirement for techniques able to create smaller structures is
ongoing. Therefore, industry and academia invest intensive
[*] D. Wouters, Prof. Dr. U. S. Schubert
Laboratory of Macromolecular Chemistry and Nanoscience
Eindhoven University of Technology and
the Dutch Polymer Institute (DPI)
P.O. Box 513, 5600 MB, Eindhoven (The Netherlands)
Fax: (+ 31)40-247-4186
E-mail: u.s.schubert@tue.nl
Prof. Dr. U. S. Schubert
Center for NanoScience, Ludwig-Maximilians-Universität München
Geschwister-Scholl Platz 1, 80333 München (Germany)
Angew. Chem. Int. Ed. 2004, 43, 2480 – 2495 DOI: 10.1002/anie.200300609 2004 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim
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Reviews
U. S. Schubert and D. Wouters
effort in developing UV and deep-UV lithographic techniques,
which can provide structural resolution down to
80 nm. [19] At the same time, particle-, ion-, and electronbeam
lithographic techniques have become available for
structuring substrates at the 50-nm level. Smaller dimensions
are possible, but the costs rise enormously and the addressable
substrate sizes decrease enormously. Both UV, particle,
and e-beam lithography are well documented, and a large
number of recent publications are available. [20–26] A publication
by Grunze et al. is of particular note because it introduces
the possibility for chemical lithography using an electron
beam. Focused electrons are used to chemically modify the
terminal nitro groups of self-assembled monolayers (SAMs)
to amines; the subsequent functionalization with carboxylic
acid anhydrides was then demonstrated (see Figure 2). [27,28]
Also of note is the preparation of ordered arrays of metallic
nanoparticles and metallic rods using the self-assembly of
metal-containing block copolymer micelles onto electronbeam-patterned
substrates, effectively combining techniques
[29, 30]
from the bottom-up and the top-down approaches.
As a result of decreasing pattern size, the demand for
microscopic characterization techniques has also increased.
Since their invention in 1982, [31] scanning tunneling microscopy
(STM) and atomic force microscopy (AFM) have
proven to be effective techniques capable of identifying the
structure and nature of surfaces and adsorbates on substrates
down to molecular and atomic resolution. The application of
AFM and STM has not been limited to inorganic semiconductor
substrates: also in polymer and supramolecular
research has their value also been demonstrated by the
elucidation of, for example, polymer phase separation and
molecular ordering. [32–34] In biochemistry, the techniques have
been applied for the determination of interaction forces
between proteins and receptors. [35–38] Soon after the invention
of AFM and the observation of its ability to modify substrates,
the field of scanning-probe-based lithography (SPL) was
developed. [39] The application of probe-based techniques for
the modification of substrates has been widespread and a
large variety of techniques have evolved, which range from
the subtle movement of atoms (using STM), [40] the formation
of local deformations in soft substrates using high-contact
force AFM (see Section 2.1) to the local application of “inks”
(dip-pen lithography) and the local oxidation of suitable
substrates (probe oxidation, see Sections 2.2 and 2.3). With
Figure 2. Electron-induced chemical lithography using self-assembled
monolayers (SAMs) as a platform for the generation of patterns. Scanning
force microscopy images show the selective binding of acetic acid
anhydride (R 1 = CH 3 ) and perfluorobutyric acid (R 2 = C 3 F 7 ) onto a SAM
platform. [27]
the application of probe-based techniques, control over
position and direction is evident. Moreover, due to the very
recently developed multi-probe systems (see Section 2.3) and
Daan Wouters studied chemical engineering
(1995–2000) at the Eindhoven University of
Technology (The Netherlands). During this
period he spent 4 months at DSM Desotech
(Elgin, USA) on a project regarding UV-curable
coatings. He graduated in 2000 under
the supervision of Dr. P. C. Thüne (the Surface
Science group of Prof. J. W. Niemantsverdriet)
working on a surface-science model
of the Philips catalyst. Since 2001, he has
studied for a PhD with Prof. U. S. Schubert
at the same university. His research centers
on scanning probe lithography and nanostructured
surfaces.
Ulrich S. Schubert was born in 1969 (Tübingen,
Germany). He studied chemistry and
biochemistry in Frankfurt, Bayreuth, and
Richmond (USA). He obtained his PhD
(1995) under the supervision of Profs. C. D.
Eisenbach (Bayreuth) and G. R. Newkome
(Tampa). After postdoctoral training with
Prof. J.-M. Lehn (Strasbourg), he obtained
his habilitation with Prof. O. Nuyken (TU
München) in 1999, followed by a year at
the Center for NanoScience (LMU, München).
Since June 2000 he has been a Full
Professor at the Eindhoven University of
Technology (Chair for Macromolecular
Chemistry and Nanoscience).
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automated scanning probe equipment, [41] the patterning of
large areas has become accessible. Finally, not only patterns
on substrates can be created, but also chemical modification
reactions on specific substrates are accessible, which allow the
combination of techniques from both the top-down and the
bottom-up approaches. Bearing this in mind, lithographic
techniques based on scanning probe techniques are the
method of choice for applications in nanotechnology. [42]
This Review provides an overview of current probe
lithographic techniques and their subsequent (chemical)
modification processes, which result in the creation of functional
structures. The field of probe lithography can be split
into two main areas: chemical and physical surface modification.
The latter includes the formation of patterns by the
physical movement of material on a substrate (for example,
by applying a high contact force or through modifications
induced by locally applied heat). Moreover, dip-pen lithography,
in which atoms and molecules attached by adhesion to
a tip can be transferred to a substrate in a controlled fashion,
is also described in this context, since in this process chemical
bonds are neither created or broken. The category of
chemical modification processes include experiments that
use local oxidation processes. This process was first observed
in STM experiments, but has also been transferred to AFM
and can be applied to conducting (metallic) substrates as well
as to thin layers of nonconducting (organic) resists.
For many of the classes in the physical modification
category, a number of publications and some applications
have been presented, and many good reviews are already
available. For this reason, only a short description will be
presented here. Moreover, no attempt will be made to present
a comprehensive overview; rather a number of typical
examples and applications of subsequent chemical modification
will be shown. The second part provides examples of the
electrical modification of substrates and organic resists using
both STM and AFM techniques. Since STM techniques are
rather well documented, this Review focuses on the application
of conductive AFM tips in the formation of functional
nanostructured surfaces.
2. Physical Probe Lithography Techniques
2.1. Mechanical Surface Patterning
Since its invention, the goal of AFM has been to image
surfaces without causing surface damage. However, the
imaging of soft (organic) materials in contact mode (with
relatively hard tips and stiff cantilevers) leads to surface
damage. Therefore contact-mode AFM for these kind of
samples is usually displaced by noncontact or intermittent
contact AFM to reduce tip-induced surface damage. The
concept of the controlled mechanical deformation of substrates
using standard AFM tips can be transferred to almost
any substrate. For example, the controlled application of
(soft) tip-crashes in STM led to the formation of patterned
substrates. The limiting factor in creating reproducible
patterns is the stability of the tip itself, which is prone to
deformation and contamination. To prevent excessive deformation,
several groups have used diamond or diamondcoated
tips. Mechanical patterning (force lithography) has
successfully been applied to substrates and films of soft metals
such as copper, gold, nickel, and silver. [43–47] The structures
that have been created include thin lines and pits. Figure 3a
Figure 3. a) AFM image (2.5 ” 2.5 mm 2 , z-range = 10 nm) of a resist
(Shipley SP25) patterned with an intermittent-contact SPM. The patterning
was performed in vector mode; the dark lines are plowed
grooves while the bright lines are embankments due to the displaced
resist; [46] b) AFM image (2.5 ” 2.6 mm 2 ) of a resonance-SPM modification
of a polycarbonate film on silicon in which the patterning force
has been applied according to greyscale bitmap image. [48]
shows an example in which a force-induced pattern was
inscribed into a photoresist film by high-force intermittentcontact
scanning probe microscopy (SPM). High-force AFM
has also been applied to polymer substrates and polycarbonate
films (see Figure 3b), both in contact as well as
intermittent operation. [46–48] Besides the mechanically induced
deformation of substrates and films, scanning probe techniques
can also be used for the gentle movement of metallic [49–51]
or latex [52] nanoparticles into dimers, trimers, linear structures,
and letters. The same method has also been applied for the
local removal of parts of SAMs and Langmuir–Blodgett (LB)
films. [53] If these experiments are carried out in a solution
containing a competing molecule or nanoparticle the free
space created by the tip will be filled in situ. Therefore, stable,
ordered, mixed monolayers or a special arrangement of
nanoparticles can be produced. Liu et al. presented an
example of the mixed-monolayer method, where monolayers
of 1-octadecanethiol and 11-sulfanyl-1-undecanol on gold
were utilized as a substrate and ocatdecyltrichlorosilane
(OTS) as a reagent in subsequent positive and negative
pattern-transfer reactions (Figure 4). The same group also
demonstrated the possibility of creating holes in SAMs of
long-chain alkane thiols on gold surfaces, into which gold
nanoparticles with a shell containing free thiols could be
deposited. [54]
Atomic manipulation is perhaps the most appealing form
of modification from a scientific viewpoint. In 1990, Eigler
and co-workers exhibited the possibility of creating artificial
structures by the direct movement of adsorbed atoms on a
metal surface in an ultrahigh vacuum (UHV) at low temperatures.
[55] A notable example of a working logic device
consisting of molecular cascades developed at IBM was also
recently presented by Eigler. [56] By arranging adsorbed CO
molecules on a Cu(111) substrate and using the instability of
bent trimer adsorbates, working AND and OR logic circuits
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Reviews
U. S. Schubert and D. Wouters
2.2. Dip-Pen Lithography
Figure 4. Schematic diagrams and corresponding AFM images
(600 ” 600 nm 2 ) of a series of experiments for the construction of 3D
nanostructures. Nanopatterns of 1-octadecanethiol (b) are grafted into
a matrix of 11-sulfanyl-1-undecanol/Au(111) SAMs (a). The hydroxyl
end groups within the nanostructure react with octadecyltrichlorosilane.
The nanopatterning results in a negative pattern transfer (c). [53]
The group of Mirkin has pioneered a patterning technique
that relies on material being transferred from an AFM tip to
the surface, known as dip-pen nanolithography (DPN). This
material can simply be the constituents of the tip itself (i.e.,
gold), induced by force or current, or it can be physisorbed
material. Material on the tip is transferred to the substrate by
capillary forces. Using DPN, Dravid et al. [57] have patterned
silicon substrates with various dyes, whereas Rayment et al. [58]
applied up to fifth-generation dendrimers (G5 DAB, amino
terminated) to Si(100) substrates to form several micrometerlong
lines with diameters up to 100 nm. The effective
obtainable line widths were highly dependent on the utilized
writing speed and temperature. [59] Although studies suggested
that the transport of material was dependent on a water
meniscus between the tip and the substrate, [60–66] writing
experiments have also been performed at zero relative
humidity, which indicates surface diffusion as being a transport
factor. [59,67] Mirkin et al. have used DPN extensively to
deposit proteins on surfaces. Recently the formation of a
nanoarray of proteins on a gold substrate has been
reported. [68] In this study, proteins (IgG and Lysozome)
were positioned in nanoarrays (islands with diameters of
200 nm) and subsequently coupled with antibodies (anti-
IgG). Figure 6 schematically demonstrates the procedure.
were combined into a two-input sorter composed of a setup of
198 molecules on a 9 ” 9 nm area (Figure 5).
Figure 6. Schematic representation of protein nanoassembly: In the
first step, dip-pen lithography was used to create 16-sulfanyl-hexadecanoic
acid (HMA) islands on a gold substrate (left). Rabbit-IgG proteins
were adsorbed onto these islands (center), followed by coupling with
anti-IgG (right). The process could be followed by observing increases
in the height profiles of AFM images after each step. [68]
The same group have utilized oligonucleotides, polystyrene
latex particles (with diameters of 190 nm and 1 mm), [66,69]
magnetic Fe 3 O 4 nanoparticles, [70] and metal-precursor-based
inks in order to create patterns down to 45 nm. Nanostructures
of Al 2 O 3 , SiO 2 , and SnO 2 have successfully been
prepared from dip-pen depositing solutions of the corresponding
MCl n -salts followed by hydrolysis to the corresponding
oxide. [71] Dip-pen lithography also allows the
creation of structures with more than one ink. By switching
to another tip, it is possible to functionalize an area next to a
previously prepared structure with another ink. [72] A review
by Mirkin et al. supplies a more comprehensive overview on
this topic. [73]
Figure 5. STM image (12 ” 17 nm 2 ) of CO molecules adsorbed on a
Cu(111) crystal at 5 K arranged into a three-input sorter. By moving a
CO molecule with the STM tip (at one or more of the inputs) into an
unstable trimer configuration, a molecular cascade is triggered (see
inset) that travels down to the output where the STM was used to
read out the result of the operation. [56]
2.3. Thermal Patterning with Multi-Tip Systems
The idea of using thermomechanic patterning for data
storage was developed by Mamin and Rugar at IBM
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Almaden. [74] Using a laser diode to heat a tip that is in contact
with a poly(methyl methacrylate) (PMMA) film to a temperature
higher than the softening point of PMMA creates an
indentation in the film that has the shape of the tip. In this
way, densities of up to 30 Gbin 2 were achieved. Although the
size of the features was initially 100 nm, writing and readout
was reliable (a system with a single tip traveling a total of
16 km in 145 h reading 200 nm marks with a 400 nm period
has been reported). [75,76] Practical data storage applications
were envisioned with the development of sharper tips, higher
readout frequency cantilevers, integrated heating, and
smoother substrates. Initial tips that had the disadvantage of
laser-diode heating were replaced by tips with integrated
heating circuits. [75, 77,78] However, mechanical limitations in the
resonant frequencies of the cantilevers restricted the obtainable
data transfer rates to a few Mb s 1 . [76, 79] Patterning speed
as well as read-out speed could be significantly increased by
the development of multi-tip systems. [80–88] To the best of our
knowledge, the group of Quate (Stanford) first reported the
use of a multi-tip system for lithography experiments on
amorphous silicon in 1995. [87] The system had five parallel tips
(Figure 7a) and was based on an earlier development by
Tortonese. The group fabricated piezoresistive cantilevers
that could operate without external sensing requirements due
to integrated force sensing. Because the tip arrays did not
allow for individual z-movement, the arrays had to be aligned
Figure 7. a) SEM image of an assembly combining five AFM tips for
parallel operation, as developed by Quate et al; b) an AFM image
(400 ” 100 mm) of a pattern obtained by the parallel operation of four
AFM tips; c) Pattern created by the parallel operation of two AFM tips
(by local-probe oxidation of Si, see Section 3). The result demonstrates
the limitation of individual z-control: the AFM image on the right
(deflection controlled by a laser/detector system) shows the expected
pattern, whereas the left tip (assumed to be in contact) performed
poorly. [87]
manually to the substrate. Utilizing this setup, 400 ” 100 mm
parallel images were obtained in constant-height mode
(Figure 7b). In a multiple-tip-scanner setup either an increase
in scan size or in scan speed proportional to the number of tips
could be realized. The lack of an individual height control
limited the applicability of the system for lithographic
purposes to a total of two tips operated in parallel. In
Figure 7c a lithographic experiment using two paralleloperated
tips demonstrates the problems associated with the
lack of individual z-control: only one of the two tips was
constantly in contact during patterning, while the other tip
produced an incomplete pattern.
In 1998, Vettiger and co-workers (IBM, Zürich) reported
a 5 ” 5-tip array with integrated force sensing and its
successful application for imaging. For stable operation, the
tip array and the substrate had to be precisely levelled. This
was realized by detecting the deflection of three cantilevers
on three edges of the chip and by control through a setup of
actuators on the array and the sample. [85] Moreover, Quate
et al. reported in 1998 a centimeter-scale AFM [84] that used up
to 50 parallel cantilevers with integrated force sensors. Surface
areas of 2 ” 2 mm were imaged at a 0.4 mm pixel size in
30 min with 10 parallel tips. An array of 32 tips was used to
study 1.28 mm 2 of a diffraction grating and an array of
50 parallel tips was utilized for lithography on hydrogenpassivated
Si(100) by electric-field-enhanced oxidation at
15 V. The resulting thin oxide lines were subsequently used as
an etching mask resulting in a 1 cm 2 patterned area with lines
that were 1.1 mm wide.
By combining the original idea of Rugar and that of a
multi-tip system, an AFM-based data storage system capable
of producing high data density and improved read-out speeds
became reality. In 2000, and again in 2003, Vettiger, Binnig
and co-workers reported the development of the so-called
“Millipede” system (Figure 8). [81–83] The “Millipede” consists
of an array of 1024 tips (32 ” 32) with integrated read/write
capabilities using combined effects of contact force and
applied heat (Figure 8a). The process of writing bits into thin
PMMA films was achieved by heating the tips to 4008C using
a current traveling through a highly doped section at the end
of the cantilever. This allowed the prestressed lever structure
to flex into the polymer (Figure 8b). Erasing large areas of
written data required heating the whole medium to 1508C,
which causes the PMMA film to undergo thermal reflow.
Individual bits could be erased by forming a second pit
adjacent to the first pit. Readout of the data was achieved by
scanning a warm tip (3008C) over the sample and measuring
the heat resistance over the bits: when a tip entered a hole, the
cantilever came closer to the PMMA film, thus increasing the
heat conductivity.
By combining 1024 tips on a 3 ” 3 mm chip, patterns with a
12 mm pitch and 40 nm data bits corresponding to aerial
densities of 400 Gbin 2 , were obtained. The first prototypes
of 4096-tip arrays (64 ” 64; 6.4 mm 2 ) were already reported [83]
and design parameters are now focused on commercializing
the concept. The “Millipede” system will probably be the first
real commercial launch of an AFM-based “nanostorage
device” as long as issues of cost reduction and long-term
operation can be resolved. [83]
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In addition to the aforementioned
techniques, probe oxidation is a promising
approach to create structures that can
be utilized in subsequent modification
steps. In this case, local surface oxidation
introduces the chemical (and physical)
functionality of the corresponding substrate.
Figure 8. a) The “Millipede” concept developed by IBM consists of an array of 32 ” 32 tips. Each tip
can be individually heated by applying an electric current through a highly doped area in the cantilever;
b) when the heated tips were pressed against a thin PMMA film, small holes were created. These holes
can be used as bits for data-storage applications. [82]
Research into multi-tip systems has not been limited to
storage applications. Progress in parallel dip-pen lithography
with an eight-pen nanoplotter was reported by Mirkin and
Hong in 2000. [80] The operating principle is similar to that of
the “Millipede”, but in this case an eight-tip array has been
used to deposit octadecylthiol on gold in eight identical
patterns. [80] In an inverted approach, Lee demonstrated the
application of a tipless cantilever and an array of inverted tips
on a substrate for application in force distance measurements.
[89] Difficulties in conventional force measurements
involving the placement of addressable active individual
molecules on a single tip could be reduced by this approach.
All of the probe-based lithographic techniques described
above do not chemically modify the substrate. Functionality is
only obtained through the mechanical movement of atoms
and molecules. Therefore no chemical modifications are
introduced that may be used in subsequent steps. Surfacemodification
processes using tip-induced force are accompanied
by a significant problem in the form of tip wear. This is in
contrast to the heat-induced phase changes in thin polymer
films (like in the “Millipede” project), where tip wear on the
medium-to-long-term time scale (months) has been shown to
be small. In contrast to mechanical and/or thermal patterning,
dip-pen lithography does not modify the substrate but adds
functionality by depositing molecules. These molecules introduce
the possibility of secondary modifications using, for
example, proteins. Although modification with dip-pen techniques
is rather slow, both due to limited ink capacity and
relatively slow writing speed, progress can be envisaged
through the development of multi-tip approaches. [80–88]
3. Oxidative Probe Lithography
Techniques
3.1. Scanning Tunneling Microscopy
Patterning
Soon after the invention of scanning
tunneling microscopy (STM) by G.
Binnig and H. Rohrer in 1982, [31]
researchers observed the ability of STM
to modify surfaces, in addition to the
effects of pure surface imaging. By 1985
Güntherodt et al. had reported the possibility
of creating structures in Pd 81 Si 19
substrates (Figure 9). [39] Structure formation
was attributed to the polymerization
of an adsorbed oil layer originating from
the vacuum oil-pumps.
In electrochemical patterning experiments,
STM can be used in two distinct emission modes. In the
first mode, electrons from the tip (negative bias on the tip) are
used to locally oxidize the material below the tip; a type of
low-energy (tunneling) electron beam induces local oxidation
of the material. In the second method (the field emitting
mode), the area below the tip is oxidized by the electric field,
while material between the tip (e.g., oxygen or water)
Figure 9. SEM image of freshly drawn lines on glassy Pd 81 Si 19 . The
lines were drawn by applying a bias voltage to the STM tip (see text).
The effect was ascribed to the local electron-induced polymerization of
oil fragments, which originated from the vacuum pump system. [39]
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decomposes and their reaction products oxidize the substrate.
A general reaction scheme proposed by Sugimura and
Nakagiri describes the anodic reactions [Eq. (1)] and
[Eq. (2)] that occur at the substrate surface and the cathodic
reaction [Eq. (3)] that occurs at the tip (see also Figure 10): [101]
M þ x H 2 O ! MO x þ 2 x H þ þ 2 x e
2H 2 O ! O 2 þ 4H þ þ 4e
2H 2 O þ 2e ! H 2 þ 2OH ð3Þ
ð1Þ
ð2Þ
Snow reported the application of this technique in order to
create one of the first working devices, initially using a silicon
tip and cantilever in an experiment oxidizing thin (7 nm)
titanium films. [106] Using a poorly conducting tip no current
could be detected, which implied a field-effect induced
oxidation of titanium to titanium oxide with adsorbed
water. By partly oxidizing thin titanium wires produced by
conventional optical techniques, thin titanium wires (10 nm)
and metal-oxide junctions were prepared (Figure 11). The
I–V characteristics of a 10 nm constricted titanium wire at
room temperature and at 4.2 K were compared to those of a
premodified (thick) wire.
Figure 10. Schematic representation of the tip-induced oxidation of
surfaces. The substrate was oxidized by an anodic reaction with the
surface water layer. At the tip a cathodic reaction generates gaseous
hydrogen.
An example of the field emission method is the direct
oxidation of a hydrogen-terminated Si(100) substrate. Currently,
this represents the most frequently studied system,
together with the direct modification of layered substrates
(i.e., HOPG or MoS 2 ) or metal surfaces (i.e., Ti or Cr);
oxidation experiments of organic resists on substrates have
been reported less often.
Heinzmann et al. have reported patterning on organic
SAMs. [90,91] In this case, hexadecanethiol and N-biphenylthiol
on Au(111) and octadecyltrichlorosilane on Si(100) were
utilized. A pattern could be etched into the silicon substrate
using two wet-etch steps (5% HF, 30% KOH). Earlier
experiments by McCord and Pease also described the
patterning of PMMA and alkyl halide resists. [92,93] They also
prepared the first working device (a thin-film resistor) by
patterning a PMMA resist, followed by a lift-off technique.
[93, 94] A comprehensive review covering the early work
in STM-induced surface modification was published in 1990
by Shedd and Russell. [95]
3.2. AFM Probe Lithography
3.2.1. Oxidation of Metallic and Semiconducting Substrates
In the early 1990s, successful lithography experiments
with STM led to several groups transferring oxidation
techniques to AFM. Field-effect induced oxidation has been
applied to Si, [96–98] Ti, [87,99] Ta, [100] and Cr. [101] Electron-induced
oxidation has been applied to organic resists, [102,103] SAMs, [104]
and LB films. [105] As early as 1995, the group of Campbell and
Figure 11. a) I--V characteristics of a thick Ti wire and a Ti wire with a
constriction of < 10 nm, measured at 300 and 4.2 K; b) AFM image of
a 120 nm Ti wire with a narrow constriction. [106]
Also in 1995, a metal-oxide semiconductor field-effect
transistor (MOSFET) was produced using AFM lithography
to create electrodes at the gate level, with the remaining
structure prepared by mask lithography. [107] The process was
confirmed with a-Si films on silicon dioxide, silicon nitride,
sapphire, and chromium. Using a tip bias of 12 V (negative at
the tip) and patterning speeds of 0.55 mms 1 , a MOSFET with
an effective channel length of 0.1 mm was formed (with a
patterning width of 0.21 mm). With transconductances of
279 msmm 1 , the electron-transport properties of the device
were comparable to those produced by electron-beam
techniques.
In 1999, Avouris et al. reported the preparation of a
single-electron transistor from constricted thin metal wires
(Ti and Nb) by means of the local probe oxidation of a thin
metal film and a process entitled current induced local
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oxidation (CILO). [108] First a thin metal wire was locally
oxidized to constrict the wire to 100–200-nm-wide gaps. Then
a transistor was produced by applying a 2 V bias over the
constricted metal nanowire while measuring the current over
the wire. Using CILO, a 10–50 nm metal-oxide barrier was
formed across the constriction of wire in a self-limiting
fashion without the need of external control. The experiments
suggested that the barrier formation involved current-induced
atomic rearrangements and local heating. Using both the
probe oxidation and the CILO process, T-shaped junctions of
two tunneling barriers with differing widths and a metal island
were produced (Figure 12).
a width of 20 nm at the base and 7 nm at the top were
produced. Packed patterns of lines with periodicities of 13 nm
were also prepared (Figure 13).
Figure 13. a and b) Local-probe oxidation of Si to SiO 2 by applying a
sequence of 21 V pulses for 1 ms to create a set of 23 interdigitated
lines placed 43 nm apart; c) a set of 19 lines placed 13 nm apart was
created by applying a potential of 24.7 V for 80 ms. [110]
Figure 12. a and b) 1.5 mm thick Ti wire with a narrow gap at the
center that was produced by local probe oxidation; c) I–V curves
across the device before and after the creation of the barrier;
d) current-induced local oxidation (CILO) created a nanometer-scale
barrier. [108]
In 1995, Quate and co-workers demonstrated probe
lithography with 40 nm resolution at very high tip speed. [109]
Speeds of 1 mms 1 were obtained on Si substrates coated with
thin films of a siloxene, known as “spin-on glass” (SOG).
SOG is used as a positive resist, and the etch speed of the
underlying silicon depends on the amount of organic content
in the SOG, which is reduced by the field-induced oxidation.
The direct oxidation of hydrogen-passivated Si substrates
can also be used in the formation of nanostructured functional
patterns. In 1995, Sugimura and Nakagiri reported a method
in which the created SiO 2 patterns were used as negative
resists in an alkaline etching step that resulted in 30 nm
steps. [101] The substrate with the same SiO 2 patterns could also
be covered with a gold film. By using electroless plating, gold
was deposited on the Si–H surface and not on the insulating
silicon oxide patterns. Recently, García et al. determined the
linewidth and shape of SiO 2 on silicon. [110] For this purpose,
several high-resolution patterns were produced on silicon
wafers with a 2 nm coating of a native oxide by applying 24 V
pulses through an n-doped tip; trapezoidial-shaped lines with
Lyuksyutov and co-workers have investigated tip–surface
interactions during the oxidation of an Si–H surface. They
reported an approach for monitoring tip–surface interactions
based on power spectral analysis of tip oscillations during
scanning probe oxidation. [111] Utilizing a single-mode harmonic
oscillator model, the amplitude, frequency, and damping
factor of oscillation were monitored as a function of bias
voltage. The results obtained were consistent with the concept
that a water meniscus and electrostatic tip–surface interactions
dominate contact AFM lithography, that is, a high bias
voltage destabilized the water meniscus and the oscillation
amplitude was reduced.
3.2.2. Oxidation of Substrates with Organic Resist Layers
The previous examples have described the direct oxidation
of Si to SiO 2 . This process can also be achieved with Si
samples that are covered with an organic resist. Experiments
utilizing resists consisting of (mixed) LB films [112] and (mixed)
SAMs [113–117] have been reported. In the first case, the
application of mixed LB films of hexadecylamine (HAD)
and palmitic acid (PA) was reported by Lee et al. Different
ratios of the LB components were used and it was concluded
that the mixed layers led to significantly thinner pattern
widths. Although lines that were approximately three times
thinner than those prepared by alternative techniques were
observed, there were no comments concerning the shape and
condition of the tip that was employed. Liu et al. reported a
method to create assemblies of gold nanoparticles on
patterned SAMs of OTS on SiO 2 /Si (Figure 14). [113] By locally
oxidizing the silicon under the organic resist, they created
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Figure 14. Functionalization method introduced by Liu et al.: In the
first step a Si wafer with an OTS resist layer was locally oxidized to
create protruding SiO 2 dots (top right). A second silane (APTMS) was
then coupled to the dots to introduce amine functionalities exclusively
to the oxidized areas. The amine-functionalized areas could be used to
bind negatively charged gold nanoparticles (bottom right). If the dots
were very close to each other, the amine pattern was not fully covered
by gold particles, which was ascribed to mutual repulsion between the
gold particles themselves. [113]
arrays or SiO 2 dots with diameters of 15 nm and a 40 nm
spacing. The protruding SiO 2 was used in a subsequent
chemical modification step in order to introduce new
functionalities. In this step a second silane (aminopropyltrimethoxysilane
(APTMS)) reacted with the SiO 2 created by
the probe oxidation, whereas the unoxidized OTS resist layer
protected the SiO 2 below. The created SiO 2 pattern was
transferred and terminal amino groups were introduced to
adsorb gold nanoparticles. The successful preparation of a
site-selective assembly of 15 nm gold nanoparticles on 100 nm
AFM-defined dots with 300 nm spacing was reported. When
an array of 30 nm dots with 70 nm spacing was utilized as a
template, the site-selective adsorption of gold nanoparticles
was incomplete: large portions of the template remained
unoccupied. This was attributed to electrostatic repulsion
between individual gold nanoparticles. Takai and co-workers
also utilized SAMs of OTS on silicon wafers to create SiO 2
nanostructures by local tip-induced oxidation processes. [114,115]
In addition, they also reported a functionalization method by
introducing a fluoroalkyl silane (FAS, heptadecafluoro-
1,1,2,2-tetrahydro-decyl-1-trimethoxysilane). [115] Due to the
large molecular dipole that is introduced, Kelvin-probe force
microscopy could be used to study the created nanopatterns.
In an earlier publication, Takai and Nakagiri reported the
successful application of the same technique to create nanostructured
gold patterns on surface. [116] First a silicon wafer
with an alkyl silane was locally oxidized to create SiO 2
nanopatterns. The SiO 2 was then removed in a 0.5% HF
etching step. Finally, gold was introduced to the patterns in an
electroless plating bath. Successful deposition of gold onto the
structures with 100 nm spatial resolution was shown by SEM
imaging.
In 2002, Takai and co-workers explored parameter space
in the oxidation of Si. [114] In this study, relationships between
the lateral and vertical dimensions of oxide dots, formed on
octadecyltrimethoxysilane(ODS)-passivated Si, and the
applied bias voltage as well as bias duration time were
reported. Based on the results, a stepwise oxidation reaction
was proposed: In the first step the organic resist was locally
oxidized and/or destroyed. As the SAM layer degrades,
anodization of the underlying silicon becomes apparent, as
indicated by an observed increase in the lateral force contrast.
For the preparation of line structures, a minimum line width
of around 20 nm has been found, which is independent of
oxidation voltage and probe oxidation speed.
The oxidative removal of SAMs has also been studied for
octadecanethiol on gold by Uosaki et al., who reported that
thiols could be removed from the gold substrate by applying a
2–3 V positive or negative bias voltage, with both the tip and
the substrate placed in toluene containing varying concentrations
of water. [118] The created structures did not consist of
bare gold, because only a 1 nm height change could be
observed. The remaining nanometer of material was ascribed
to adsorbed reaction products of the thiol–gold cleavage,
which is consistent with the measured resistivity (1.7 ” 10 9 W)
for a 1 nm resist on gold.
In addition to the oxidation of Si to SiO 2 and the
subsequent functionalization reactions, the oxidation of
molybdenum to molybdenum oxide by local probe-oxidation
techniques was also reported by Dai and co-workers. This
approach represents another successful route for the preparation
of devices that then allows subsequent modification
steps. [119] The principle of functionalization in this approach is
based on the different solubility of Mo and MoO 3 . A thin
layer of Mo on a Si substrate was locally oxidized. The
produced MoO 3 could easily be removed by dissolving it in
water and therefore exposing the bare Si substrate underneath.
The exposed silicon was wet-etched by a 30 % aqueous
solution of KOH at 708C for 30 s, which yielded V-shaped
grooves in the underlying silicon.
Titanium wires were prepared by using a more elaborate
functionalization technique. [119] A thin film of Mo was applied
on top of a PMMA film on Si. After the MoO 3 was removed,
the PMMA film was locally removed by oxygen plasma
etching, which opened a pathway to the silicon wafer for Ti
deposition. The remaining Mo film protected the unexposed
areas. By this technique, Ti wires (35 nm wide) were prepared
(see Figure 15). The same group also suggested the application
of chromium and germanium films based on the solubility
of the corresponding metal oxides.
3.2.3. Oxidation of Organic Resist Layers: Introducing Chemical
Functionality
All of the functionalization processes mentioned so far
have been based on the direct oxidation of substrates or the
destruction of a thin passive resist layer on an oxidizable
substrate. Although complex structures have mainly been
prepared using wet etching steps, and in some cases by silane
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Figure 15. A Mo film (4 nm) was evaporated on top of a PMMA film
that was spin-coated onto a p-doped Si substrate. a) The Mo layer was
locally oxidized (7–11 V, 0.4–1 mms 1 ) by local-probe oxidation to
create a slightly protruding soluble MoO 3 pattern with 35 nm wide
lines; b) in the second step, the MoO 3 was dissolved in water to create
25 nm wide gaps that exposed the underlying PMMA film; c) the
exposed PMMA film was then etched by exposure to oxygen, to create
deep channels; d) in the last step, Ti was evaporated onto the whole
substrate. Removal of the PMMA film by a lift-off procedure (using
acetone) yielded Ti nanowires on the remaining Si support. [119]
chemistry, the range of applicable functionalization techniques
is limited. A notable exception to destructive functionalization
was reported by Sagiv et al., who found that the tipinduced
oxidation of organic monolayers introduced surface
functionality, which could then be used in a large number of
subsequent functionalization reactions with both organic and
inorganic substances yielding a range of nanopatterned
functional structures. [120–124] The work involved is summarized
in Scheme 1. The procedure was first demonstrated on
monolayers of 18-nonadecenyltrichlorosilane (NTS) on a p-
doped silicon wafer (see Scheme 1, path 1). [121] The terminal
vinylic end group could be locally oxidized by a conductive
AFM tip with an applied bias voltage of approximately 9 V
(tip negative). Oxidation was carried out at 5 mms 1 with a
variety of conductive tips (boron doped, diamond coated,
tungsten, tungsten carbide, copper- and silver coated silicon
nitride, and highly doped silicon). Structures were obtained
by repeatedly (up to 40 times) scanning the tip along a
predefined vector during which no current (above 5 nA)
could be detected. The successful selective oxidation of the
terminal vinylic end groups to terminal carboxylic acid groups
was confirmed by subsequently imaging the area in contact
mode with the same probe (without applying a voltage),
simultaneously recording height and lateral friction signals.
For the unaffected areas the surface remained unchanged,
whereas for the oxidized areas the height remained the same
but an increase in friction was observed, which was attributed
to changes in local surface polarity. Images of this patterning
procedure are displayed in Figures 16 and 17 in which this
procedure is used for subsequent modification reactions.
Oxidation experiments were verified by partial wet-chemical
oxidation: the substrate was partially exposed to a 5 mm
solution of KMnO 4 /dicyclohexano-[18]crown-6 and also to
electric oxidation of large surface areas by means of a copper
grid (connected to a 13 V negative bias voltage). Brewster
angle infrared spectroscopy indicated the presence of carboxylic
acid groups.
To prevent oxidation of the underlying silicon it is
essential that the surface is covered by a densely packed
closed monolayer. By controlling the pathway of the tip over
the surface, complicated structures with high resolution (9 nm
linewidth) could be created. For this procedure both vectorbased
trajectories as well as bitmap (pixel per pixel) oxidation
programs were reported. In their first publication, Sagiv et al.
reported the possibility of functionalizing structures using
chemical reactions that react exclusively on the oxidized
patterns. [121] The concept was demonstrated by the formation
of a second stable monolayer of OTS on top of the oxidized
sections of the 18-nonadecenyl layer. The coupling of the
second monolayer was followed by contact-mode AFM
imaging. Through the coupling of OTS to the oxidized
areas, the terminal carboxylic end groups were converted to
terminal methyl end groups. This was observed in contactmode
imaging by the disappearance of the friction signal
along with a simultaneous increase in the height signal
(Figure 16). [121,123]
Later publications showed that not only NTS SAMs could
be used but also inert SAMs of OTS could locally be oxidized
to carboxylic acid end groups (Scheme 1, path 2), thus
opening a route for chemical modification. [123] Noteworthy
is the approach in which a monolayer of NTS was attached to
locally oxidized OTS (Scheme 1). Thus functionalized patterns
have been used as templates for the formation of
metallic layers by using wet-oxidation and subsequent
adsorption and reduction of metal ions (Scheme 1, path 2a).
The terminal vinyl groups could also be converted to thiol
groups by UV irradiation (Hg lamp, 254 nm) for 10 min in an
H 2 S/Ar (1:1) atmosphere. As a result a stable pattern
decorated with thiol end groups placed in well-defined
positions was created, and could be used in subsequent
functionalization steps. Stable layers of metals could be
formed on the templates by reduction of suitable physisorbed
metal ions (either tip-induced or by wet chemistry). Application
of gold nanoparticles (Au 55 ) to the thiol groups resulted
in the formation of gold islands connected by lines that were
only two nanoparticles wide (Figure 17). [120] The method
presented by Sagiv et al. for surface patterning and modification
has many possibilities for the production of nanometersized
devices as a result of the large number of available
chemical and physical modification steps, for example using
silane chemistry (silanes with terminal functionality), the
coupling of amines and cationic species, thiol chemistry, and
conversion to anhydrides. [125]
We have performed surface modifications on OTSfunctionalized
Si wafers using a method similar to that
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Scheme 1. Functionalization method developed by Sagiv et al. 1) A monolayer of 18-nonadecenyltrichlorosilane (NTS) on a Si wafer was locally
oxidized. Chemical functionalities were introduced that were used as templates for subsequent modification steps, such as the attachment of a
second (different) chlorosilane; 2) the oxidation procedure was also applied to octadecyltrichlorosilane (OTS) monolayers, introducing similar
functionalities. Wafers were prepared with terminal methyl groups (from OTS) patterned with local terminal vinylic functionality end groups. NTS
was coupled to local-probed oxidation-induced patterns. The local presence of vinyl groups was utilized in two different pathways: 2a) through
chemical oxidiation to carboxylic acid functionalities, and 2b) in which the vinyl group was converted to a thiol group. Both functional patterns
were used in various subsequent steps including the coupling of gold particles and the formation of metallic silver and cadmium sulfide islands.
Figure 16. AFM images showing the formation of a NTS-functionalized
template structure (see also Scheme 1a). a) After tip-induced inscription
of a pattern of electrooxidized OTS (OTSeo); b) Images of an
NTS/OTS bilayer formed by exposure of the patterned OTS monolayer
to a 5 mm solution of NTS in biscyclohexane (BCH). [123]
developed by Sagiv et al. By utilizing a number of conductive
AFM tips with various coatings (Au, TiN, Pt, and W 2 C) and a
broad range of stiffnesses (0.15–5 N m 1 ), an OTS monolayer
was locally oxidized to create templates down to 20 nm
resolution, which were then used in a number of different
subsequent functionalization steps. [126,127] Figure 18 shows the
Figure 17. Topography (left) and phase-contrast (right) AFM images
showing portions of gold-nanoparticle-coated lines produced by localprobe
oxidation (see also Scheme 1, path 2b). Using this method,
both wires and contact pads were produced. [120]
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results of the specific adsorption of trimethyloctadecylammonium
bromide (TOA). In the first step, the OTS monolayer
was locally oxidized to create carboxylic acid groups. The
local height remained almost constant after oxidation
(Dz max = 0.5 nm) and when measured in contact mode a
clear friction signal originating from the different surface
properties of the unmodified OTS and the carboxylic acid
groups was observed (Figure 18a and b). Upon adsorption of
Figure 19. Tapping-mode height image (1.1 ” 1.1 mm, z-range 18 nm)
of cationic gold nanoparticles adsorbed onto an oxidized structure.
The observed height of the particles ranges from 17 to 20 nm.
Successful experiments have also been performed in which
functional silanes were coupled to the oxidized patterns. Two
silanes, one with a terminal vinyl functionality and one with a
terminal methylmethacrylate group (both targets for free
radical polymerization reactions) were coupled to the template.
In both cases the polymer was grafted to the surface
species; the method appears to be promising due to the wide
availability of functional silanes.
3.3. Surface Modification by Switching Tip Properties
Figure 18. a) Contact-mode height image (8 ” 14 mm, z-range 0.5 nm)
of an oxidized triangle; b) friction image of the same triangular shape,
indicating that successful oxidation had occurred; c) height image
(8 ” 14 mm, z-range 0.5 nm) after exposing the surface to a solution of
trimethyloctadecylammonium bromide (TOA) in water, which causes
the height of the triangular area to increase by 2.0 nm and indicates
the adsorption of one layer of TOA; d) the corresponding friction
image; e) contact-mode height image (30 ” 30 mm) of a structure created
by a sequential (six-step) method using both oxidation and the
adhesion of TOA.
the TOA from an aqueous solution, a height step (D = 1.5 nm)
was observed, which corresponds to the length of the
octadecyl chain; simultaneously the friction signal disappeared
due to the reformation of an apolar top surface
(Figure 18 c and d). The created structures were stable when
washed with water. The oxidation/adsorption procedure was
repeated three times, each time introducing a structure next
to the previous one, but never destroying the original pattern
(Figure 18 e).
Besides the adsorption of quaternary ammonium salts, the
procedure has also been demonstrated for positively charged
gold nanoparticles (d = 15 nm, coated with poly-l-lysine;
Figure 19). The protein Candida Antarctica Lipase B was
selected for protein-coupling experiments as it contains thio
functionalites on its outer surface. Initial results showed the
successful coupling of proteins to a gold template. [128] Possible
applications may be found in nanosensor array devices.
Ober et al. [129] introduced a technique called redox probe
microscopy (RPM), in which an AFM tip was modified with
redox-active materials. When a potential was applied, the tipsurface
interaction could be switched.
Two methods of lithographic operation were presented:
The first is based on controlling the adhesion between the tip
and the particles on the surface, effectively picking them up
and positioning them in a pattern (see Figure 20). For this
procedure the tip was coated with poly(vinylferrocene)
(PVF), which is neutral 0 V, but is oxidized to the ferricenium
state when the bias is increased (+ 1.0 V), thus becoming
highly positively charged. The positively charged tip strongly
interacted with negatively charged beads (15 mm, sulfonic
acid terminated chromatography beads), which could be
Figure 20. A poly(vinylferrocene)-coated tip in the reduced state is neutral.
However, when oxidized (tip bias + 1.0 V) the positively charged
tip could be used to pick up negatively charged particles. [129]
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picked up, and released again by reducing the PVF film. The
described experiments were repeated with switchable tips
consisting of a gold-coated tip modified with a 6-(ferrocenylcarbonyl)hexanethiol.
In the second method, surface modification was performed
by an AFM tip that caused a change in pH value when
an electric bias was applied (Figure 21). In this approach, a
gold-coated tip was modified with thiomethyl-2,5-hydroquinone
(thioQH 2 ). At positive biases the thioQH 2 is oxidized to
the quinone and two protons per molecule are generated. As
a result, a film of a pH-sensitive triblock copolymer (MMA-
TBMA-MAA) has been patterned. [129]
Figure 21. a) The pH lithographic method: The oxidation of thiomethyl-2,5-hydroquine
releases protons that can be used to locally pattern
a pH-sensitive block copolymer film of methyl methacrylate
(MMA), tert-butyl methyl methacrylate (TBMA) and methyl acrylic acid
(MAA); b) AFM images of the block copolymer MMA-TBMA-MAA (A,
left) showing the effects of tip-induced surface damage due to the
repeated scanning (30 min) of a 500 ” 500 nm area. Image B (right)
shows a similar approach where repeated scanning over an area was
performed with the AFM tip cycled between oxidative and reductive
modes. Note the enhancement in the generated pattern. [129]
4. Conclusions and Outlook
UVand UV-mask lithographic processes are currently the
most widely employed and studied techniques for the
production of sub-micrometer-sized devices. However, no
dramatic improvements in the accessible sizes should be
expected in the future. Although good progress has been
made in field of electron-beam lithography, the conditions
remain very demanding, which leads to excessive costs.
With this in mind, scanning-probe-based lithographic
techniques seem more feasible. Within this field, research
efforts into dip-pen lithographic techniques have led to welldefined
systems capable of depositing (biological) materials
onto flat substrates at a relatively high speed. With the
development of multi-ink systems, applications into biorelated
sensors are envisioned in much the same way as
devices already available that are based on microcontact
printing. However, dip-pen lithography patterning is limited
due to the low availability of suitable ink combinations, the
limited stability of the patterning due to diffusion, and the low
patterning speed. The patterning speed can be increased
dramatically by the introduction of multiple-tip systems, such
as those developed by IBM for AFM-based storage systems.
The simultaneous operation of 1024 tips allows reasonable
reading and writing speeds to be obtained. In combination
with automated AFM systems and sample stages, the
patterning of large substrates then becomes possible.
Probe-induced oxidative techniques may also become
more viable. Such techniques have the advantage of being
relatively easy, with respect to the required conditions and
equipment. Even more importantly, the method is highly
versatile both in applicable substrates as well as in the
availability of functionalization routes. Local oxidation can be
performed on almost any conducting sample, even on thin
organic resists, at high resolution. The technique can be
employed to locally oxidize the sample and resist layers, thus
inducing changes in physical properties. The obtained systems
can be employed in both positive as well as negative
development of the pattern by various wet-etching techniques.
By this technique, a number of small metallic nanowires
and nanometer-sized electronic systems have been prepared.
Besides development by etching procedures, local oxidation
can also be used for chemical modification steps on organic
resists. Chemical functionalities such as thiols, amines, or
carboxylic acids can be introduced into defined patterns.
Thus, a large number of modification routes are possible,
which open up new avenues for the preparation of nanometer-sized
functional architectures on surfaces. Applications
can be envisaged in various areas, ranging from the defined
introduction of biological material (for example, DNA or
proteins) for sensor applications, to the coupling of polymerization
initiators or catalyst species for mechanistic studies
and the preparation of special smart coatings or intelligent
polymer brushes.
The Dutch Polymer Institute (DPI) and the Fonds der
Chemischen Industrie are acknowledged for their financial
support.
Received: May 26, 2003 [A609]
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