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Materials & Design 243 (2024) 113036
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0264-1275/© 2024 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Crafting at the nanoscale: A comprehensive review of mechanical Atomic
force microscopy-based lithography methods and their evolution
Lorenzo Vincenti
a
,
*
, Paolo Pellegrino
a
,
b
,
c
,
*
, Mariafrancesca Cascione
a
,
b
, Valeria De Matteis
a
,
b
,
Isabella Farella
b
, Fabio Quaranta
b
, Rosaria Rinaldi
a
,
b
a
Department of Mathematics and Physics “Ennio De Giorgi”, University of Salento, Via Monteroni, 73100 Lecce, Italy
b
Institute for Microelectronics and Microsystems (IMM), CNR, Via Monteroni, 73100 Lecce, Italy
c
Istituti Clinici Scientici Maugeri IRCCS of Telese Terme Institute, 82037 Telese Terme, Italy
ARTICLE INFO
Keywords:
Mechanical-Scanning Probe-based Lithography
Atomic Force Microscopy nanolithography
Atomic Force Microscopy
Nanomachining
Nanofabrication
Nanomanipulation
ABSTRACT
In recent years, the scientic community’s interest in nanoscience and nanotechnology stems from the increasing
capability to manipulate matter at the nanoscale. Nanotechnology development is closely linked to fabricating
and characterizing structures below 100 nm, driven by technological advancements enabling their in-depth
analysis. Up to now, several top-down and bottom-up nanofabrication approaches have been developed to
realize a plethora of nanostructures. Although effective, these methods have many drawbacks like high costs and
limitations in feature size. In this scenario, Scanning Probe-based Lithography (SPL) emerges as a very promising
alternative to conventional nanofabrication techniques, overcoming their main method limitations with versa-
tility, exibility, low cost, and nanoscale resolution. This review focuses on mechanical Scanning Probe-based
Lithography (m-SPL), tracing its evolution from inception to recent advances. Different m-SPL methods, such
as Nanoindentation, Static and Dynamic Plowing lithography, Nanomilling, and their variants are discussed in-
depth, emphasizing their advantages and drawbacks, and highlighting their application. Moreover, this review
explores the effects of combining m-SPL with other energy sources, such as heat and electric energy, and outlines
future perspectives in the eld. Overall, m-SPL stands out as a promising avenue in nanofabrication, offering sub-
nanometer resolution and diverse material manipulation capabilities.
1. Introduction
The enthusiasm of the scientic community toward nanoscience and
nanotechnology is rapidly increasing, as well as the capability to
manipulate materials at the nanoscale is a common interest to most of
the technology’s elds, supported by proven applications and potential
future implications. The development of nanotechnology is closely
related to the capability to fabricate and characterize structures whose
sizes are less than 100 nm in one or more dimensions [1,2]. Driven by
the technological progress that allows in-depth analysis and manipula-
tion of materials at the nanoscale [3,4], the nanomaterials found
applications in many academic and industrial research areas. Since the
invention of the integrated circuit by J. Kilby in 1959, the efforts of
researchers have been focused on the development of innovative, high-
throughput technological approaches to realize nanostructures and
nanopatterns that are massively employed in the semiconductor in-
dustry [5] and, in particular, in electronics. In this eld, the main issues
concern data storage, scalability, and costs involved in the
manufacturing process. However, the discovery of new, heterogeneous
properties of nanomaterials, together with the progress in the nano-
fabrication methods, have permitted the successful employment of
nanostructures in a plethora of elds, i.e. nanouidics [6,7],
Abbreviations: AFM, Atomic Force Microscopy; CP-AFL, Constant Pulse-Atomic Force Lithography; DPL, Dynamic Plowing Lithography; EBL, Electron-Beam
Lithography; ELD, electroless deposition; FIB, Focused Ion Beam Lithography; GP-AFL, Gradient Pulse-Atomic Force Lithography; LAO, Local Anodic Oxidation; m-
SPL, mechanical-Scanning Probe-based Lithography; NIL, NanoImprinting Lithography; NTIL, Nanotip Indentation Lithography; P-AFL, Pulse-Atomic Force
Lithography; PMMA, poly(methyl methacrylate); RIE, Reactive Ion Etching; SAM, Self-Assembled Monolayer; SATP, Standard Ambient Temperature and Pressure;
SPL, Scanning Probe-based Lithography; SPM, Scanning Probe Microscopy; STM, Scanning Tunneling Microscopy; TPL, Two-Photon Lithography; TPP, two-photon
polymerization; t-SPL, thermal-Scanning Probe Lithography; UNCD, ultrananocrystalline diamond.
* Corresponding authors at: Department of Mathematics and Physics “Ennio De Giorgi”, University of Salento, Via Monteroni, 73100 Lecce, Italy (P. Pellegrino).
E-mail addresses: lorenzo.vincenti@unisalento.it (L. Vincenti), paolopellegrino@unisalento.it (P. Pellegrino).
Contents lists available at ScienceDirect
Materials & Design
journal homepage: www.elsevier.com/locate/matdes
https://doi.org/10.1016/j.matdes.2024.113036
Received 9 April 2024; Received in revised form 9 May 2024; Accepted 20 May 2024
Materials & Design 243 (2024) 113036
2
nanoelectronics [8–10], nano-optics [11], medicine [12], drug delivery
[13], and the development of biosensors [14], antimicrobial surfaces
[15,16], nano-scaffolds [17], and optical devices [18]. Indeed, the
growing demand for nanostructures and nanostructured materials
further supports the improvement of conventional nanofabrication
techniques and the development of new strategies for the manipulation
of a wide range of materials at the nanoscale [19–21]. In particular, the
use of semiconductors, polymers, dielectrics, conductors, and organic
materials is essential to lead the development of many elds of tech-
nology, whose perspectives could be strongly affected by nano-
technology’s achievements.
To date, the major top-down approaches adopted in the nano-
fabrication eld are photolithography [22] Electron-Beam Lithography
(EBL) [23–25], Focused Ion Beam Lithography (FIB) [26], Two-Photon
Lithography (TPL) [27,28], and NanoImprinting Lithography (NIL)
[21]. EBL is a maskless nanolithography technique consisting of a
focused electron beam that impresses an electronic resist layer. Then,
the effectiveness in nanopatterning different materials is reached
through the resist-developing process [24]. Although EBL is a very ac-
curate and effective nanofabrication method, the high cost of the
equipment, the slow etching rate, the necessity for further process steps,
and the proximity effects make this technique unsuitable for repetitive
serial production in manufacturing industrial applications [24,25].
Presently, EBL is mainly employed for manufacturing masks for optical
lithography, and in academic research [29,24,25]. On the other hand,
FIB is an accurate micro/nanofabrication, both top-down and bottom-
up, method in which a focused ion beam is used to pattern nano-
structures on several substrates directly. FIB has been used for decades
to fabricate complex 3D micro-/nano- structures, easily and precisely.
Indeed, it is known that FIB is not a awless technique. However, the
costs of the equipment and its maintenance are very high; extremely
reliable ion beam sources and highly competent personnel are required.
Differently, NIL is a molding process based on the use of a nanopatterned
mask and an imprint liquid, usually a polymer, for manufacturing
nanostructures on a substrate. The mask is in contact with the imprint
liquid before it solidies, thus achieving a one-to-one ratio transfer of
the mask’s features to the polymeric material, and nally to the sub-
strate through an etching process. Although this technique allows a
direct patterning of the nanostructures, it requires a proper mask, made
using another method. In addition, the NIL method is not suitable for
constructing complex 3D nanostructures [21,30]. Another effective,
bottom-up, maskless nanofabrication method is the Two-Photon Poly-
merization (TPP). It owes its name to the non-linear absorption process
involved in suitable materials and is used for manufacturing 3D free-
standing micro/nanostructures beyond the diffraction limit [27,28].
A valid alternative to the nanofabrication techniques previously
described is represented by Scanning Probe-based Lithography (SPL),
with its wide range of different approaches, recently developed to
overcome the limitation of the conventional nanofabrication methods, i.
e. the cost, complexity, and efciency for academic research and in-
dustrial applications [31–33]. The development of SPL approaches has
directly followed the probe microscopy methods, which alone have
brought about major advances in nanotechnology in the last decades
[8,34].
Initially, the pioneering Scanning Tunneling Microscopy (STM)
technique was developed in 1982, as a powerful exploitation of the
quantum phenomena involving electron tunneling. Subsequently, in
1986, Binning et al. introduced Atomic Force Microscopy (AFM) [35].
AFM is a high-resolution technique which operates by leveraging the
interaction between the sample’s surface and a probe, inuenced by
either Pauli repulsion or van der Waals forces.
In AFM systems, a laser beam is focused on the cantilever apex and
then reected by its surface on a four-sectional photodiode, whose
current response is recorded and used for implementing a feedback loop
that drives the scanner or the probe, according to the instrument spe-
cics [36]. This technique achieves atomic resolution, but this strongly
depends on the sharpness of the tip [37].
The invention of Scanning Probe Microscopy (SPM) techniques, and
the successive development of SPL, paved the way to a new paradigm of
conceiving the manipulation of material at the nanoscale, that was un-
thinkable before. For instance, the AFM capability to interact with a
material can be fully exploited by AFM-based lithography techniques,
which represent the focus of this review. The instrument setting and the
experimental setup, indeed, have signicant advantages concerning the
conventional nanofabrication method. First, the AFM tip can be
employed as a nanotool to directly pattern the sample surface [38]; then,
the patterned nanostructures can be immediately characterized using
the same tip, avoiding further additional steps for characterization,
which are required for other nanofabrication techniques. Moreover, the
experimental setup itself is simple, as an AFM instrument can usually
perform at the Standard Ambient Temperature and Pressure (SATP)
conditions.
In the last decades, SPL approaches have been extensively exploited
in both academic and industrial research elds because of their versa-
tility, exibility, low cost, accuracy, and nanoscale resolution
[32,39,40]. To date, several types of SPL methods have been developed
such as Local Anodic Oxidation (LAO) [40] or thermal [41], electric
[42], dip-pen [43], and mechanical lithography [44]. In this framework,
mechanical-SPL (m-SPL) is recently emerging as a very promising
approach in the SPL domain, since it allows the manipulation of mate-
rials with a sub-nanometer resolution by applying a wide range of force
on the sample surface, according to different operations modes [45,46].
Moreover, additional energy sources can be provided to the tip to
enhance the fabrication process quality or to make it able to pattern
specic materials.
This review intends to present a thorough overview and the current
state of mechanical Scanning Probe-based Lithography (m-SPL)
methods. It covers the evolution of these methods from their inception to
the latest rened techniques, providing insights into their fabrication
mechanisms. The advantages and drawbacks of the different techniques
are in-depth illustrated to allow a comparison among different m-SPL
approaches. Representative applications of m-SPL techniques are pre-
sented to point out their potential and critical issues. The effects of
combining m-SPL with other forms of energy, for example, heat and
electric energy, are discussed and future perspectives are suggested.
2. Nanoindentation and related lithography techniques
2.1. Nanoindentation
The more straightforward m-SPL technique is nanoindentation,
which relies on the employment of the AFM probe to realize nanoholes
or arrays of nanoholes on several materials. In nanoindentation, the tip
is brought in contact with the sample surface and the probe can be
further pushed toward the sample, thus causing the tip to indent the
material (Fig. 1 a and b). Typically, the curvature radius of the AFM tips
at the apex ranges from a few to tens of nm, and the interested area on
the sample is in the order of hundreds of nm
2
wide or less. Moreover, the
tip penetrates for a depth of less than a hundred nm, and this is the
reason why this technique is named nanoindentation.
Different theoretical models have been developed to describe the
interaction between the material and the probe, which depends on the
tip geometry and on the sample’s material properties. When AFM
nanoindentation is performed with a spherical-shaped tip and the
indentation depth is smaller than the tip radius, the Hertz model is used
to t the data relating Young’s modulus of the material to the load
applied to the indenter [47], as it describes the indentation of an in-
nitely hard, spherical indenter on an elastic cylinder [48]. For inden-
tation depths comparable to or exceeding the radius of curvature of the
probe, the Sneddon model better describes the interaction between an
innitely hard conical indenter and an elastic cylinder [48]. Other
models, based on these ones, were built to t the presence of a pyramidal
L. Vincenti et al.
Materials & Design 243 (2024) 113036
3
tip [47] or a different shape of the sample [49]. In this framework, the
efforts made by G. M. Pharr, W. C. Oliver, and F. R. Brotzen resulted in a
formula that can be applied when a rigid, axisymmetric punch indent an
elastic half-space [47,50,51].
In 2007, N. M. Pugno developed a more sophisticated theoretical
model than those previously formulated to estimate the hardness of
materials through AFM-based nanoindentation [52]. The proposed
model was based on the surface-to-volume ratio of the domain in which
the energy ux occurs, considering the different sizes and shapes of the
indenter [52]. The law derived from this approach had the previously
known laws as limit cases, but it was able to t the deviation from these
models towards the nanoscale [52]. Afterward, this model was further
developed, focusing on the effect related to the tip radius of curvature
[53]. In order to test the model, three AFM silicon probes with different
tip corner angles were nanofabricated, and nanoindentations were
performed on soft material. The tip curvature radius is considered by
introducing a correction factor to the hardness value obtained by a
model developed for an ideal tip, which would be characterized by a
Fig. 1. AFM-based Nanoindentation technique for the nanopatterning of nanohole array. (a) Scheme of AFM Nanoindentation operation mode. (b) Graph describing
the movement of the AFM probe in the x ,y, and z plane during the nanoindentation. (c) On the top panel, arrays of nanoholes generated through nanoindentation
onto a 350 nm thick photoresist layer, utilizing a scanner extension, that is the variation of the scanner’s height position, of 100 nm for indentation. In the bottom
graph, the variation in the nanoholes depth (squares) and diameter (circles), alongside the bulge width (triangles), was plotted as a function of the scanner extension.
L. Vincenti et al.
Materials & Design 243 (2024) 113036
4
vanishing curvature radius. Then, the correction factor is applied to the
experimental results to nd the actual material hardness value [53]. Up
to date, nanoindentation is further employed to determine the local
properties of a huge range of materials, such as hardness, Young’s
modulus, and adhesion forces [51,54,55].
In addition to material hardness estimation, nanoindentation can be
employed to plastically deform the material when an adequate force is
applied. Indeed, the nanoindentation can create a nanohole on the
material surface, whose geometry is determined by the tip’s shape, by
the normal force applied by the probe on the material’s surface, and by
the material’s intrinsic characteristics. During the nanoindentation, the
tip is subject to an increasing force due to the cantilever’s bending,
which is detected by the photodiode and monitored by an oscilloscope.
The maximum force value and the time for a single indentation are the
two parameters to be carefully set in this technique.
On the right side, nanohole grids patterned on Silicon after Reactive
Ion Etching (RIE) process resulting from AFM nanoindentation on a
resist, with scanner extensions of 70, 80, 90, and 100 nm, by means of
Electron Beam Deposited AFM tip. Reprinted with permission from
Ref. [56]. Copyright 2000 AIP Publishing LLC. (d) Schematic repre-
sentation of the AFM indentation lithography process, followed by wet
etching procedure and metal deposition, for the patterning of plasmonic
nanostructures. On the bottom side images, both the nanoindentations
on the mask layer and the corresponding metallic nanostructures were
characterized by AFM and SEM. Scale bars correspond to 200 nm. Bright
eld optical microscopy images of eight nanostructures arrays (1–8)
prepared using a single AFM tip and SEM images of the selected area
with 4000×magnication (middle) and 10000 ×magnication (right).
The scale bars are 2
μ
m and 500 nm, respectively. Reprinted with
permission from Ref. [11] Copyright © The Royal Society of Chemistry
Fig. 2. Constat Pulse- and Gradient Pulse- Atomic Force Nanolithography Techniques. Sketch representation of (a) Constant Pulse and (b) Gradient Pulse – AFL; the
red arrows on the AFM probes represent the force pulses used to sculpt the substrate. (c) and (d) Graphs depicting the AFM probe movement in the x ,y, and z planes
during the CP- and GP- AFL, respectively. Two-dimensional, high-resolution AFM images of (e) nanolabyrinth; (f) concentric circular-shape nanostructures; (g) array
of linear, parallel nanochannels; (h) Circular and (i) triangular, (j) linear and (k) serpentine-like nanostructures whit an increasing depth prole. The unconventional-
shape nanostructures were fabricated on a thin PMMA layer by Constant Pulse-AFL (e-g) and Gradient Pulse-AFL (h-k). Reprinted with permission from Ref. [58,59].
Copyright 2022 MDPI.
L. Vincenti et al.
Materials & Design 243 (2024) 113036
5
2021.
The efcacy of nanoindentation as a nanofabrication strategy has
been demonstrated and utilized by different studies, in particular on
polymeric materials [56,57]. Among them B. Cappella and H. Sturm
performed nanoindentations on poly(methyl methacrylate) (PMMA) at
different values of the maximum normal force applied and they found a
linear relation for depth up to about 160 nm [57]. K. Wiesauer and G.
Springholz fabricated arrays of nanoholes on a resist layer, and then
successfully transferred the pattern to the underlying Si substrate by
reactive ion etching, reaching a feature’s size of 7 nm [56] (Fig. 1. c).
More recently, J. Kim and co-workers used Nanotip Indentation
Lithography (NTIL) combined with wet etching, to realize arrays of
plasmonic nanodisks and nanotriangles array on a glass substrate. The
shape of the nanostructures depends on the shapes of the AFM tip apex:
nanodisks were realized by a conical tip while nanotriangles were
patterned by means of a pyramidal-shaped probe. In detail, the glass
substrates were coated with two layers of polymers: the shallower
polymer lm acted as a mask for the etching process while the bottom
polymer layer worked as a sacricial layer. After nanoindentation and
the subsequent deposition of a gold layer, the polymer bilayers were
removed (lift-off step) and the plasmonic nanostructures were success-
fully patterned on the glass substrate. Although this protocol may appear
complex and laborious, it guarantees excellent performances in terms of
precision, accuracy, and reproducibility. As an added bonus, the NTIL
technique prevents degradation and damage to the tip [11] (Fig. 1. d).
2.2. Pulse-atomic force Lithography
Recently, P. Pellegrino et al. proposed and optimized an alternative
AFM-based lithography approach based on the nanoindentation
method, termed Pulse-Atomic Force Lithography (P-AFL). The main idea
is that nanoholes patterned close enough would merge into a continuous
nanostructure [58,59] (Fig. 2. a-d). This technique allows the patterning
of nanochannels with sub-nanometric precision and high accuracy on a
polymeric layer. With this method, it is possible to quickly and easily
obtain 2D or 2.5D nanogrooves with the desired depth, both constant or
variable, length, and slope in a single pass and without additional energy
sources [58].
In detail, the P-AFL comes in two different variants: Constant Pulse-
Atomic Force Lithography (CP-AFL) and Gradient Pulse-Atomic Force
Lithography (GP-AFL), which enable the fabrication of nanogrooves
with constant or gradient depth proles, respectively [58]. In the latter
case, the grooves change their prole smoothly and continuously. The
difference between the CP-AFL and the GP-AFL technique is that in the
rst one the indentations are performed all with the same normal force
value, while in the second one its intensity linearly increases for suc-
cessive nanoindentations [58,60] (Fig. 2 c and d).
P-AFL techniques were employed for patterning various complex
shapes, circular, triangular, and serpentine-like on a polymer substrate
[59] (Fig. 2 e-k). Moreover, 2D and 2.5D nanostructures can be
patterned on a resist layer and transferred to the underlying, harder
material by conventional plasma etching with high delity and repro-
ducibility [58].
2.3. Plowing Lithography
The term “plowing lithography”, related to AFM-based lithography,
refers to m-SPL techniques developed from two conventional AFM im-
aging modes: contact and semicontact mode. While in AFM imaging tip
contact must leave the sample unmodied, in plowing lithography
greater normal forces are exerted on the material’s surface by the tip,
and scratches or nanogrooves are obtained due to the motion of the tip
relative to the substrate.
In the Static Plowing Lithography technique, the AFM probe is in
contact with the sample to scratch its surface by moving forward and
backward along a grid of lines with the suitable force to shape the
material (Fig. 3 a and b). In order to obtain well-dened nanostructures,
several parameters must be carefully set, such as the interaction force,
the scanning direction, the scanning rate, and the material to be
patterned has to be carefully selected [61]. A. A. Tseng et al. explored
the possibility to fabricate nanogrooves and nanodots by Static Plowing
Lithography on a 30 nm Ni
80
Fe
20
thin lm deposited on a Si substrate by
scratching the surface with a diamond-coated tip, characterized by an
apex radius between 115 nm and 125 nm, and loaded with a normal
force of 9 µN [62] (Fig. 3 c). At rst, arrays of nanogrooves of almost 15
nm in depth were patterned respectively with a pitch of 80 nm and 35
nm, that is the distance between two adjacent grooves. Then, an array of
lines was scratched on the 35 nm pitch array, respecting the same pitch
but in the perpendicular direction, thus obtaining a quasi-3D nano-
structure consisting of an array of evenly distributed hemispheric
quantum dots, which had a dot density of 2.6 ×10
8
per mm
2
[62–66]
(Fig. 3 c).
L. A. Porter et al. developed a nanofabrication process to successfully
pattern gold nanoparticles and nanowires on semiconductor Ge(1 11).
First, they spun a (20 ±2) nm photoresist layer on a substrate of Ge
(111). Then, they performed Static Plowing Lithography by using a
silicon tip loaded with a normal force in the range from 3 µN to 5 µN, and
a scan rate of 100 µm/s [61]. It resulted in nanogrooves that reached the
underlying Ge substrate. Finally, the sample was immersed in a solution
of HAuCl
4
, letting the Au deposit on the semiconductor’s exposed sur-
face, where it shaped into nanoparticles that grew and merged to form
nanowires [61]. Similarly, S. B. Ulapane et al. developed an innovative
method for the deposition of metal microstructures on different sub-
strates [67]. By using silicon glass and mica as substrates, they deposited
on them a uniform thin lm of CaCO
3
NPs, previously synthesized and
diluted to a proper concentration. To remove the resist material from
square areas on the substrate, the Static Plowing Lithography technique
was optimized by varying the scanning rate, the applied normal force,
and the number of passes. Then, a uniform lm of gold was evaporated,
and the remaining CaCO
3
NPs layer was dissolved in an aqueous solu-
tion, leaving a gold square microplatform on the substrate [67] (Fig. 3
d).
In AFM Static Plowing Lithography, a very high resolution can be
obtained by patterning Self Assembled Monolayers (SAMs) on at sub-
strates with a very sharp tip, characterized by a tip radius of the order of
the nm or less. In this framework, C. M. Edwards et al. investigated the
possibility to fabricate copper, silver, and gold nanostructures on silicon
(Si) substrates through the nanopatterning of a SAM resist by AFM
nanolithography, followed by metal deposition in the structure using
electroless deposition (ELD) [68]. The substrate doping, SAM composi-
tion, and AFM scanning parameters were optimized, and the ELD solu-
tion capability to selectively deposit metals on exposed areas of the Si
surfaces was demonstrated [68].
A further step towards AFM application at molecular resolution can
be made by working with AFM in liquid media. The SAMs imaging is
performed in contact mode under very low normal forces, to avoid
damaging the monolayer. Based on Static Plowing Lithography, two
different methods can be developed to pattern the SAM in liquid media,
depending on the desired result: nanoshaving and nanografting [69]. In
both cases, a normal force greater than the molecules’ displacement
threshold must be applied. However, while nanoshaving is completely
analog to Static Plowing Lithography as previously described, in nano-
grafting technique, different molecules are introduced in solution to
replace the SAM molecules when they are mechanically displaced by the
AFM tip [69].
Static Plowing Lithography can be used to pattern complex 2D
geometrical gures in a very short time because the scan rates could be
set up to about tens to hundreds of µm/s [61]. For asymmetrical probes,
to maintain a constant scratching direction and not affect the quality of
nanostructures with undesired effects due to the lateral force and the tip
orientation, the sample must be rotated during the scratching process
[70]. In addition to the aforementioned Static Plowing Lithography
L. Vincenti et al.
Materials & Design 243 (2024) 113036
6
drawbacks, the need for an experimental setup more elaborate than the
one used for AFM imaging led to the development of the DPL approach,
which ensures a pattern almost independent of the scanning direction,
and a reduced tip wearing during the nanofabrication process [70]. Y.
Yan and coworkers used the Static plowing lithography technique to
scratch linear patterns with different orientation, on a polycarbonate
(PC) sample surface by means of a diamond probe. In addition, they
performed two consecutive tip scans with different scratching angles, in
order to obtain complex 3D nanopits arrays. Although this technique is
very simple and fast, when operating in contact mode, the presence of
cantilever deformation is inevitable as the result of the friction force
between the tip and the sample surface. This, in turn, leads to the for-
mation of irregular nanofeatures, with an inhomogeneous prole,
severely limiting the use of this technique in the production of nano-
structures (Fig. 3 e).
As the scratching direction and the tip geometry strongly affect the
nanostructures’ depth and morphology [62,72], to overcome these
drawbacks researchers have developed Dynamic Plowing Lithography
(DPL) with which it is possible to generate nanostructures by sculpting
the sample surface in semicontact mode [57] (Fig. 4 a and b). In DPL,
the lateral force is avoided, because the AFM tip is driven to oscillate in
the vertical direction at a high frequency while scratching. Besides, the
interaction force is relatively small in the dynamic response between the
tip and the sample surface, thus only shallow nanostructures can be
generated [44,72,73]. In addition, debris from the sculpted substrate
could accumulate inside the patterned nanostructures and undesired,
remarkably high pile-ups surround the carved structures [57].
Typically, Dynamic Plowing Lithography is performed by setting the
AFM in semicontact mode and driving the probe near its resonance
frequency above the material to be patterned. The tip oscillates with an
amplitude greater than the amplitude usually set for imaging, forcing
the probe to strongly interact with the material. For this reason, the force
exerted on the material, is not constant but has a period determined by
the oscillation frequency of the cantilever. Then, the plastic deformation
of the material is mainly determined by the vertical tip’s oscillation
[72]. Compared to the scratching process of Static Plowing Lithography,
the nanostructures fabricated by the Dynamic Plowing Lithography are
contoured by pile-ups with heights comparable to that of the structures
[57]. Moreover, the formation of chips of the removed material, which is
a common drawback when Static Plowing Lithography is employed for
patterning metals, does not occur when using DPL [72].
The DPL method is suitable for patterning nanostructures of a few nm
on several type of polymers, such as poly(methyl methacrylate) (PMMA)
[46,73,75–77], polystyrene (PS) [76], polyimide [78], and poly(3-
hexylthiophene) [79]. In this framework, B. Cappella et al. investi-
gated the physical mechanism by which two different polymers, PMMA
and PS in particular, could be modied during DPL [76]. It turned out
that during the rst 20–30 contacts of the tip on a single point of the
Fig. 3. Static Plowing Lithography technique for the fabrication of complex 2D and 3D nanostructures. (a) Sketch illustration of the SPL technique; the red arrow
indicate the constant force acting by the AFM probe on the substrate. (b) Graphical representation of the movement of the AFM probe during SPL in the x ,y, and z
planes. (c) From the top to the bottom, 3D AFM topographic images of parallel nanogrooves with a pitch of 80 nm and 35 nm, and quantum dots with a diameter of
45 nm patterned by cross scratching with a pitch of 90 nm. All the nanostructures were scratched by backward direction using diamond-coated tip at contact mode,
using a normal force of 9
μ
N. Reprinted with permission from Ref. [62]. Copyright © 2011 Elsevier B.V. All rights reserved. (d) Optical images showing (50 ×50)
μ
m
2
square micropatterns realized by SPL varying the patterning parameters, such as the tip velocity, the applied force, and the number of scans. All studies were
conducted on a CaCO
3
nanoparticles (NP) layer. Reprinted with permission from Ref. [67]. Copyright © 2021 American Chemical Society. (e) 2D and 3D AFM images
and corresponding 2D FFT images of nanodot arrays patterned by SPL technique. The scratching angles used for the two-step scratching method were 90◦and 0◦, 90◦
and 45◦, and 0◦and 45◦(from the to the bottom of the image). Reprinted with permission from Ref. [71]. Copyright 2014 Springer Nature.
L. Vincenti et al.
Materials & Design 243 (2024) 113036
7
sample’s surface, the energy dissipated in the process breaks covalent
bonds of the polymer chains. A nanohole is progressively carved out
during this rst step. Then, the successive contacts do not induce the
enlargement of the hole, but the displacement of the carved material
around the hole to form pile-ups: in this case, the energy is distributed
over a larger area and it is insufcient to further break covalent bonds
[76].
During the nanopatterning procedure of a single line by DPL, the
intermittent contact between the probe and the sample surface can be
exploited in two ways: at relatively low scanning rates a single scan can
produce a nanogroove, while above a scanning rate threshold, multiple
scans can result in equally spaced nanoscale pits [80]. The scanning rate
threshold depends on a single parameter, that is the resonance frequency
of the cantilever. Y. He et al. employed an AFM equipped with a silicon
tip, characterized by a nominal spring constant of 42 N/m and resonant
frequency of 320 kHz, to nanopattern array of pits on PMMA [80]. In
this case, the authors found that the threshold rate was 100 µm/s and,
moreover, the nanoholes’ depth was strongly inuenced by the scanning
rate: as the scanning rate increased, the depth of the patterned nanohole
decreased. The relatively high scanning rate resulted in a throughput of
4800 – 5800 pits per second [80]. However, the authors found that the
geometry of the nanostructures is also inuenced by the scanning di-
rection when an asymmetrical shape tip is used. In a successive paper,
the same author explored the possibility to develop a data storage sys-
tem based on the nanopits’ manufacturing by means of the DPL. In this
system, the Boolean value “1” was assigned to the patterned nanohole,
while the “0” value was assigned to the unpatterned PMMA at an
assigned position [81]. The scanning rate was tuned to pattern eight
consecutive rods of nanoholes, then 8–bit code words can be read in the
normal direction. While the process parameters have been optimized to
obtain well-dened rows of equally spaced nanopits, some critical issues
still remain, as the non-linearities found in the relationship between the
Fig. 4. Dynamic Plowing Lithography technique for the fabrication of nanostructures, nanogrooves, and an array of nanoholes. (a) Illustration of the tip movement
and the substrate response during the DPL technique. (b) Graphical representation of the spatial movement of the AFM probe during DPL in the x, y, and z planes. (c)
On the left, a schematic representation of the tip movements in the xy plane for the nanopatterning of arrays of nanodots obtained by cross scratching in different tip
directions to obtain various array geometries. At the center, 2D and 3D AFM topography images of the nanodots arrays on PMMA. On the right, FFT images obtained
from the AFM topography data. Reprinted with permission from Ref. [46]. Copyright© 2017 Elsevier B.V. All rights reserved. (d) 3D AFM topographic images of
structures fabricated by DPL on graphene: lines, squares, circles, and the word ‘NANO’. Reprinted with permission from Ref. [74]. Copyright © 2013 IOP Pub-
lishing Ltd.
L. Vincenti et al.
Materials & Design 243 (2024) 113036
8
distance of the nanoholes and the scanning rate and the impossibility of
tuning the scanning rate while nanopatterning a single row, that results
in a repetition of the pattern [81].
As an alternative to Static Plowing Lithography, the fabrication of
nanoholes arrays can be also carried out by crossing two or more arrays
of nanogrooves obtained with the DPL technique [62,73]. The main
advantage of the use of this approach is the prevention of tip wear
because DPL is performed in semicontact mode. However, DPL results
are unsuitable for nanopatterning a wide range of materials harder than
polymers, due to the weaker interaction with the substrate, if compared
to the other mechanical AFM-based nanolithography techniques [73]. In
this framework, Y. He et al. investigated the DPL capability to pattern
arrays of dots on PMMA by crossing two or three square arrays of
nanogrooves, whose pitch (30 nm) corresponds to the distance between
two consecutive scanning lines. Checkerboard nanodots were fabricated
by cross scratching in the 30◦and 120◦directions, diamond-shaped
nanodots in the 90◦and 150◦directions, and hexagonal nanodots in
the 30◦, 90◦, and 150◦directions. In each pattern, the nanodots’ den-
sities orders of magnitude were 10
9
dots per mm
2
[46,73]. The patterned
nanogrooves were characterized by a depth of about 2 nm and a width of
15 nm, determined by the tip’s apex radius, which was less than 10 nm
(Fig. 4 c). Moreover, the authors found that the shape of the silicon tip
employed in DPL affects the nanostructures’ geometry only for scanning
directions parallel to or away from the main axis of the rectangular
cantilever; in the other cases, the nanostructures are not signicantly
affected by changing the scanning direction [46] (Fig. 4 c).
Although the major efforts to develop the DPL nanopatterning
approach have been made on polymeric materials [44], this technique
could be interestingly applied to nanostructuring of other specic ma-
terials. The manipulation of graphene at the nanoscale, indeed, can take
several advantages from the AFM tool. Aiming to a nanolithography
technique that does not need the nanofabrication of metallic contacts on
the graphene akes, B. Vasi´
c et al. investigated AFM capability to cut
and locally deform the graphene through the DPL setup [74] (Fig. 4 d).
The DPL test has been performed by a conventional AFM system
equipped with a V-shaped cantilever, which presents at its end a tip with
a 50–70 nm curvature radius [74]. At rst, seven trenches were fabri-
cated side by side, each one was patterned with an increasing interaction
force that reached a maximum value of about 68 µN, with which the
deeper nanogroove was patterned. The size and depth of the grooves
realized by the lowest forces were not measurable because the force
applied by the AFM tip was too low to produce an appreciable defor-
mation of the graphene layer, while the other grooves’ depth varied
from 1 nm to about 4 nm, and their width increased from 40 nm to 50
nm. In addition, those nanostructures were characterized by the pres-
ence of a large bulge on the right side. In the last nanogrooves, the
graphene layer was cut, displaced, and rolled at the top of the nano-
structures by the tip during the DPL process. Where the graphene has
been removed, a smooth surface, that widens from the bottom to the top
reaching a width of just a few nm, divides the trench from the pristine
graphene and represents the underlying SiO
2
/Si surface [74]. This
experimental setup and the DPL technique enable the realization of
more complex nanostructures, such as graphene nanoislands with a
diameter of tens of nm, squares, concentric circles, and the word
“NANO” (Fig. 4 d) [74].
3. Assisted m-SPL techniques
The outstanding performances of AFM, the scientic relevance of the
AFM-based analysis, its capability to be used as a nanofabrication tool,
and, above all, its relatively simple operating principle, paved the way
for the development of several m-SPL variants. In this frame, further
energy sources were added to the conventional AFM-based nano-
fabrication approaches to facilitate and improve the mechanical
removal of material. Through the integration of additional hardware
components to the conventional experimental setup, mechanical,
thermal, or electric energy forms can be provided to the AFM system,
improving the efciency of the lithography process and the quality of
the nal result. To this aim, different approaches have been explored
during the last decades and they are presented in this section.
3.1. Vibrating assisted m-SPL
As described in the previous section, the DPL is based on the AFM
tapping mode, in which the probe oscillates close to its resonant fre-
quency. The choice of this specic frequency makes the oscillation
amplitude of the probe very sensitive to the interaction between the tip
and the material and, therefore, to the variation of the distance gap
between the tip and the sample surface. Although this represents an
enormous advantage when performing AFM imaging of soft material,
such as topography and phase shift detection, in the case of DPL this
circumstance represents a disadvantage as the sensitivity of the system
limits the capability to optimize the geometric characteristics and the
uniformity of the nanostructures [82].
To overcome these drawbacks, L. Zhang and J. Dong developed the
Ultrasonic Vibration Assisted Atomic Force Microscopy (UVA AFM)
nanomachining technique [82,85]. In this approach, the tip is set to
oscillate in the vertical direction at an ultrasonic frequency (f), which is
much higher than the resonant one (fr, with f≫fr); moreover, a high
frequency circular planar motion between the tip and the sample is
added to the nanofabrication system, in order to achieve better control
over the nanofabrication process and, then, to increase the material
removal rate [82]. Indeed, when the cantilever vibrates at a frequency
below the resonant frequency (f<fr), the probe’s oscillations follow the
sample’s motion, and no signicant indentation is made. However, at
ultrasonic frequency, the cantilever’s dynamic stiffness increases as the
cube of the frequency, so the tip indents into relatively soft materials
[82,85].
To demonstrate the effectiveness of UVA AFM, a few nanometers
deep trenches have been fabricated on both aluminum and PMMA layers
by means of a silicon probe set to oscillate in the vertical direction with
an ultrasonic frequency of 3 MHz. In addition, a circular motion at a
frequency of 4 kHz in the plane of the sample was applied. In order to
study the lateral force signal and friction, the cantilever’s deection was
accurately recorded [85]. In UVA AFM, the depth of the nanopatterned
features is mainly controlled by the oscillation amplitude of the canti-
lever. As for other AFM-based nanofabrication techniques, the nano-
structures are contoured by the material removed during the
nanomachining process [85] which strongly limited the use of the
nanopatterned nanostructures. Anyway, the application of smaller
interaction forces than the other AFM-based nanofabrication techniques
(i.e. Static Plowing Lithography) allows to prevent the tip wear during
the lithography process [82,85]. J. Deng, J. Dong and P. Cohen used
UVA-AFM to successfully realize complex nanostructure with discrete
height levels and continuous changes on a thin PMMA layer (Fig. 5 c).
Moreover, the authors explored the capability of transferring the com-
plex, 3D nanostructures from polymer layer onto the underling silicon
substrate by RIE process [83].
In another research work, J. Deng and co-workers used the UVA-AFM
for realization of 3D microstructures on PMMA, further used as the
master in the 3D NIL (Fig. 5 d) [84]. A mathematical model for UVA
AFM lithography was developed by Shi et al. to determine a relation that
estimates the ultrasonic machined depth based on the tip oscillation
amplitude [86]. The theoretical representation of the system was fron-
ted according to the equivalent point-mass model of the continuous AFM
probe [87], and applying an approximation for high-frequency to the
Euler-Bernoulli beam theory [86,88]. The results of the simulations were
compatible with experimental data that were collected after machining
nanogrooves on a 5 nm thin lm of PS. In particular, the UVA AFM tool
was capable of regulating the machining depth at approximately 0.15
nm by controlling the probe oscillation amplitude [86]. The theoretical
model was further analyzed to determine the phase change when the tip
L. Vincenti et al.
Materials & Design 243 (2024) 113036
9
Fig. 5. Vibrating assisted m-SPL technique for complex 3D micro- and nano-structures fabrication. (a) Schematic representation of Vibrating assisted m-SPL,
depicting the circular movement of the AFM probe and the substrate vibration. (b) Graphical representation of the AFM probe spatial movement during the vibrating
assisted m-SPL in the x ,y, and z directions. (c) 2D and 3D view of AFM characterization of pyramidal-shaped nanostructures and their cross section. On the bottom
side, bitmap images of pyramid, cone and AFM 2D and 3D view of Steve Jobs’ prole. Reprinted with permission from Ref. [83]. Copyright: The Authors. Published
by Elsevier B.V. (d) On the left side, schemes in grayscale of 3D nanopatterns and feature dimensions of each nanopatterns (the height unit on the left side of the
image is nanometers). PMMA master template realized by Vibrating assisted m-SPL, and its cross-sectional proles on the right. PDMS mold reversely casted by using
the PMMA master template, and its cross-sectional proles on the right side. Reprinted with permission from Ref. [84]. Copyright © 2021 Elsevier B.V..
L. Vincenti et al.
Materials & Design 243 (2024) 113036
10
reaches the interface of an ultra-thin lm, then it was tested by moni-
toring in real-time the nanopatterning of the interface between a thin
lm of PS on Si [89]. The change in the slope of the depth and phase data
trend, that were collected for nanogrooves machined on an 11 nm thick
PS thin-lm, is consistent with the numerical simulation of the mathe-
matical model [89].
To better understand the role of debris in the machining process, a
similar experimental setup and a proper theoretical model were devel-
oped. The AFM phase response can sense the effect of the debris on
machined depth, even if the debris formation is random and unpre-
dictable [90].
During multiple studies, Deng et al. developed and improved the
UVA AFM 3D nanofabrication of structures on PMMA and successfully
applied this technique in the fabrication of 3D master nanotemplates for
3D soft lithography [83,84,91–95]. Essentially, UVA AFM technique
allows two approaches for 3D nanopatterning: layer-by-layer machining
and varying the set point of the normal force applied during machining.
In layer-by-layer machining, the whole nanostructure is fabricated by
machining several layers one by one but changing the patterns for each
layer [91]. The layer-by-layer machining approach was employed in the
realization of simple features on PMMA, as stair-like nanostructure with
ve steps. Moreover, complex nanostructure with discrete and contin-
uous height changes were successfully fabricated in raster scan mode
from bitmap images [83,93].
3.2. Nanomilling
As described in the last section, in the UVA AFM the tip motion
relative to the sample’s surface consists of two main movements: the
vertical vibration of the tip at an ultrasonic frequency, and a circular, in-
plane motion [82]. For UVA AFM, the main characteristic innovation
could be addressed in the rst one of these motions. Indeed, the capa-
bility of the in-plane circular motion to improve the nanostructures’
quality has been investigated by B. A. Gozen and O. B. Ozdoganlar in
2010, and it led to the development of a different lithography technique
called nanomilling [96]. Moreover, both these approaches rely on an
AFM system equipped with a three-axis piezoelectric actuator [97] that
allows the supply of out-of-phase sinusoidal excitations in mutually
perpendicular directions to obtain the circular motion of the tip relative
to the sample [96,97]. Nanomilling was performed by using two
different tip circular movements: a trajectory parallel to the plane of the
sample’s surface, called in-plane motion, and one perpendicular to it,
named out-of-plane motion [96] (Fig. 6 a and b). Although both the
basic congurations are effective in nanopattering, the in-plane
conguration was taken into more consideration because of its ability
to pattern different materials with high resolution [98,99]. The
Fig. 6. Nanomilling AFM-based Lithography technique for the fabrication of nanochannels. (a) Illustration of the tip movement on the substrate during the
nanomilling technique; the circular motion of the tip is represented by the red arrows. (b) Graphical representation of the spatial movement of the AFM probe in the
x ,y, and z directions during nanomilling procedure. (c) SEM images of the channels created using different nanotool motions. The material removal mechanism is
shearing dominated, as testied by the presence of long and curled chips, that are observed for the cases with nanotool motions. Reprinted with permission from
Ref. [96]. The Author(s) 2010. This article is published with open access at Springerlink. (d) The nanochannels can be shaped into different planar geometries by the
proper rotation of the substrate. Reprinted with permission from Ref. [96]. The Author(s) 2010. This article is published with open access at Springerlink.
L. Vincenti et al.
Materials & Design 243 (2024) 113036
11
material-removing mechanism results from the combination of the cir-
cular movement with the motion along the feeding direction at a pre-
scribed depth resulting in the material-removing process [96].
B. Arda Gozen and O. Burak Ozdoganlar used AFM nanomilling for
the realization of channels with a length of 5 µm and 100 nm depth by
using each of the in-plane and out-of-plane congurations on SU-98
resist (Fig. 6 c) [96]. After this preliminary optimization step, the au-
thors realized complex 3D nanostructures, such as circular, triangular
and square nanostructures by both in-plane and out-of-plane congu-
rations (Fig. 6 d) [96].
AFM equipped with silicon or diamond probes has been suitable for
patterning 2D/3D nanostructures on polymeric materials, such as
PMMA [97,100,101], metals like copper [98,102,103], aluminum alloys
[99], silicon [98,104], and other customed materials [105]. A typical
characteristic that is found in nanomilling is the formation of chips of
the removed material rather than pile-ups or debris; as a consequence,
the edges of nanogrooves and other nanostructures patterned by this
technique are well dened compared to the conventional AFM
scratching approach [96]. Moreover, the machined nanotrenches width
can be easily tuned by setting the maximum amplitude of the driving
voltage that determines the diameter of the circular trajectory, allowing
to fabricate a nanogroove of chosen dimensions in a single pass,
regardless of the tip radius [96].
The relevance of this technique was proven by a deep study of the
wearing of ultrananocrystalline diamond (UNCD) AFM tips when
nanomilling silicon and copper [98]. The tip was set to rotate in-plane
with a frequency of 4 kHz and a radius of 100 nm. Trenches of an ex-
pected depth of 150 nm were patterned on {10 0}-silicon along the
〈110〉direction, and on copper. The tip wearing was monitored by
measuring the probe’s topography at different stages of the nano-
fabrication process, corresponding to increasing patterned length. This
experiment shows how the tip undergoes rapid wear during the rst mm
for silicon, and 40 mm for copper, then it reaches a steady-state wear
phase, where the wear rates are reduced to a relatively constant value
measured to be 0.135 nm/mm for copper, and 0.19 nm/mm for silicon
[98].
The nanomilling approach has been validated by the modeling of the
technique [99–101] and Molecular Dynamics (MD) simulations
[102–104], that accounted for the tip geometry and trajectory in the
manufacturing of different materials. Due to the cyclic tip trajectory,
two scratching processes could be distinguished in a single loop: the tip
cuts the pristine material along the scanning direction, thus creating the
outer prole of the nanostructures; during the remaining part of the
trajectory, the tip avoids the untouched material and renes the inner
part of the nanostructures, enhancing their depth [101]. The direction of
rotation of the tip, which can be clockwise or counterclockwise, as well
as its geometry, determines the chip formation and affects the nano-
structures’ morphology [100,101,104].
J. Wang et al. established a theoretical model to predict the
machined depth of nanochannel manufactured on single-crystal silicon
by implementing the AFM nanomilling technique and using a triangular
pyramidal diamond tip [106]. The model was tested, and the system
parameters, including the feeding direction and the crystal orientation
of the sample, were optimized to fabricate well-dened nanochannels.
The nanostructures were characterized by means of Raman Spectros-
copy and.
Transmission Electron Microscopy (TEM) to determine how the
nanofabrication process affects the crystallographic order of silicon:
stacking faults, dislocations, and other atomic-scale defects occur at the
interface between the bulk material and the surface that has been
exposed to the high pressure and shear stress [106].
Nanomilling has been successfully employed in the nanopatterning
of multi-layer metallic lm [105] and in the manufacturing of 3D
nanostructures by the modulation of the normal force applied during the
process [99]. Moreover, the chip formation represents an advantage for
debris removing and after process treatments, because easy and effective
removal strategies could be adopted, while these are not suitable for the
elimination of debris and pile-ups formed in conventional scratching
methods [99].
3.3. Thermal-assisted SPL material removal
A particular variant of Scanning Probe Lithography (SPL), known as
thermal Scanning Probe Lithography (t-SPL), has recently regained
prominence due to its speed and reliability. In this method, localized
material modications are induced using thermal energy generated by a
heated tip (Fig. 7 a and b). Indeed, the AFM probe can be heated up to
800 ◦C by employing a laser beam [41,107,108] or a micrometric
electric resistance [41,109]. In this way, the tip represents a heat source
that can be used in different applications, such as improving the material
removal in mechanical fabrication processes, the determination of a
local chemical change of the 2D material substrates, and the transfer of
material to the sample’s surface through a physic or chemical process
thermally activated. The tip heating is achieved through the current ow
in a specially shaped cantilever, usually composed of doped single-
crystal silicon [110], low-doped Si [111–113], or a combination of
highly doped conductive cantilever with a low-doped region at the top,
where the tip is located [114]. The cantilever can be represented as a
conductive resistive element, which allows the application of a voltage
and the resistive tip heating through the Joule effect [114]. t-SPL has
proved to successfully nanopattern different materials, i.e. pentaery-
thritol tetranitrate (PETN) [110], PMMA [115], Molecular Glass (MG)
photoresists [111,116,117], and poly(phthalaldehyde) (PPA)
[118,119].
Generally speaking, t-SPL techniques were categorized into the
removal, conversion, and addition of material. The main application of
t-SPL involves localized material removal using a heated tip to alter the
surface topography. Two distinct processes are employed: mass-
preserving thermomechanical indentation, where the material is dis-
placed to the sides of each indentation, and permanent removal of ma-
terial from the surface through sublimation. The latter is a captivating
process with which it is possible to remove material by sublimation: in
this case, the high-temperature tip causes the activation of a chemical
reaction that has gaseous products, so the material can be removed
without leaving any residual debris on the sample [116].
Nonetheless, this section is focused on the mass-preserving thermo-
mechanical indentation method, in which the m-SPL techniques are
coupled with a high-temperature heated tip. Polymers are particularly
suitable materials to exploit in this method because of their physico-
chemical properties, i.e. the glass transition, or the enhancement of
chemical reactivity at high temperatures. The coupling of high tem-
perature with the application of mechanical force leads to an increase in
the efciency of the material removal process and the improvement of
the quality of the patterned nanostructures is achieved.
Recently, S. Chang and colleagues used the t-SPL technique with the
aim of manipulating a polymethyl methacrylate thin lm at the nano-
scale through a single scratch. The authors conducted a systematic
analysis on the correlation between machining performance and
scratching velocity, considering variable tip thermal conditions while
maintaining a uniform normal force. Moreover, the theoretical frame-
work of stick–slip friction was applied to rationalize the genesis of
nanopit architectures (Fig. 7 c) [120].
O. Coulembier and co-workers exploited t-SPL to pattern large areas
of a polymeric lm with high throughput and resolution. The t-SPL
method is showcased through a debris-free polymer decomposition re-
action activated by the close proximity of a heated probe (Fig. 7 d)
[118].
Y. K. R. Cho et al. methodically investigated the t-SPL nano-
fabrication parameters to improve the technique’s resolution. Silicon
cantilevers were employed to pattern nanogrooves on a 6–7 nm thick
layer of PPA on a silicon substrate [121]. The temperature of the resis-
tive heater in correspondence with the tip was studied in the range from
L. Vincenti et al.
Materials & Design 243 (2024) 113036
12
Fig. 7. Thermal-assisted SPL technique. (a) Sketch representation of the Thermal-assisted SPL nanofabrication method and (b) graphic describing the spatial
movement of the AFM probe in the x ,y, and z directions during procedure thermal-assisted SPL technique. The red arrow indicates the force applied by the tip to the
sample surface while the red shadow around the AFM tip represents the increase of the tip temperature. (c) AFM characterization of 10 µm long nanogrooves arrays
obtained by thermomechanical nanolithography under different coupling of tip temperature (T
tip
) and velocity (v
tip
) on PMMA. In details, the v
tip
values were set to
0.1, 0.5, 1.0, 1.7, 2.5, 3.4, 4.4, 5.5, 6.7, 8, 20, 40, 80, 128, 188, 260, and 500
μ
m/s (from left to right in AFM images) while the T
tip
was 110, 140, 170, 200, and
230 ◦C while the normal force applied was kept constant and equal to 250 nN. Reprinted with permission from Ref. [120]. Copyright © 2021 The Author(s).
Published by Elsevier Ltd. (d) AFM acquisition of a patterned (18 ×18)
μ
m
2
area by means of thermal-assisted SPL technique. Inset bottom left of the image on the
top: optical image of the nanopatterned area. On the right side, close-up view of the sample area highlighted with a red box in the AFM image and cross-section
prole (blue line) taken along blue cross section line in image above. Reprinted with permission from Ref. [118]. Copyright © 2009 American Chemical Society.
L. Vincenti et al.
Materials & Design 243 (2024) 113036
13
525 ◦C to 800 ◦C in steps of 25 ◦C, and for each temperature, the effect of
the normal force applied was studied from 0 nN to 30 nN. The tip radius
at the apex was about 3 nm, and the best results have been obtained with
a temperature of approximately 600 ◦C [121]. After the t-SPL nano-
patterning, the nanogrooves were transferred on the underlying silicon
layer by a dry-etching process. The optimization of parameters has made
possible the nanofabrication of 14 nm half-pitch lines in silicon with a
feature size of 7 nm [121].
Moreover, a paper by P. Vettiger et al. [122] demonstrated that the
development of a system composed of thousands of probes operating in
parallel for nanofabrication or imaging was possible. In particular, they
fabricated an array of 32 ×32 AFM probes that was capable of parallel t-
SPL nanoindentation on PMMA. This system was developed by IBM
company for high-density memory Storage and reading [122].
To conclude, this nanofabrication technique has signicant potential
and has recently been applied to overcome the limitations of conven-
tional techniques for the realization of components that can be inte-
grated into the next generations of technological devices. Relevant
examples of these components are optical Fourier surfaces for optical
devices [123], metal contacts fabricated on monolayer MoS
2
exhibiting
vanishing Schottky barriers for top-gate and back-gate eld-effect
transistors [124], and nanopatterned biocompatible silk broin to con-
trol the growth of conductive laments for the implementation of reli-
able and ultrafast memristors [125].
3.4. Bias-assisted m-SPL
According to specic characterization techniques such as Kelvin
probe force microscopy (KPFM) and Electric Force Microscopy (EFM)
[126], conductive tips can be used to study the electric properties of the
sample’s surface by setting a proper electric potential. The application of
an electric potential between the tip and metallic, conductive sample
surface was further exploited to develop other AFM-based nano-
lithography techniques (Fig. 8 a and b) [42]. Overviewing those
different approaches, deposition techniques in gaseous or liquid envi-
ronments have been developed from the capability to control electro-
chemical reactions in the proximity of the tip, and these approaches go
under the name of Local Anodic Oxidation (LAO) [127,128,129]. LAO
allows the manufacturing of nanostructures, oxides for example, on
different materials, such as silicon [128,129], Diamond-like Carbon
(DLC) lm [129], and graphite [130]. On the other hand, AFM Elec-
trochemical Nanomachining (ECM) was used to directly nano-
patterning, surface planarizing, and fabricating of 3D-ECM. The key
point of ECM is to conne electrochemical reactions at the micro/nano-
meter scale. For example, Bark et al. carried out the lithography of line
patterns of various widths and thicknesses on NbS
2
thin lms with ECM
by varying scan speed and applied voltage.
G. Lee et al. have explored the advantages of activating electro-
chemical reactions through ultra-short potential pulses during a me-
chanical scratching nanofabrication process on Cu [131]. The
conductive probe was immersed in an electrolyte solution, obtained by
combining HClO
4
0.1 mM and CuSO
4
0.01 mM. The sign and magnitude
of the potential difference between the tip and the sample, the duration
and frequency of the potential pulses, as well as the scanning speed of
the tip on the sample inuenced the deposition rate of copper from the
sample to the tip or vice versa [131]. For this reason, the shape of the
structures depends on the scratching and biasing voltage pulse param-
eters, and nanogrooves can appear surrounded by pile-ups or not, based
on the pulse amplitude. In particular, the width of the nanogrooves
tends to be almost proportional to the pulse amplitude and duration, and
inversely proportional to the tip scanning rate [131] (Fig. 8 c).
Similarly, Y. Yang and W. Zhao investigated the effects of coupling
the AFM scratching method with a voltage bias between the tip and a
sample made of a thin lm of copper to manufacture 2.5 D nano-
structures [42]. To better understand the effects of coupling, the same
nanostructures were rst patterned with an unbiased tip, i.e. purely
mechanical scratching, and, successively, the same structures were
patterned with the biased tip alone, without applying any normal force
on the sample [42]. This study demonstrates that the 2.5D nano-
structures machined on copper by the coupling AFM lithography were
deeper and smoother with respect to the ones patterned by the AFM
mechanical lithography and the AFM electric lithography. Moreover,
the authors demonstrated the existence of a potential threshold for
copper, beyond which material removal processes are activated due to
the presence of an electric discharge, resulting in a shock wave and local
heating of the sample where the tip is located. Moreover, this strategy
was successful for the nanopatterning of platinum, while the mechanical
scratching alone failed [42] (Fig. 8 d).
4. Challenges and perspectives
In this review, we reported the state of the art of the mechanical-
–Scanning Probe-based Lithography techniques, describing in-depth
their main advantages and disadvantages, together with their practical
applications in several nanotechnology elds and future perspectives.
First, AFM provides high-resolution imaging and manipulation ca-
pabilities at the nanoscale, enabling the creation of intricate nano-
structures with unprecedented precision. Its versatility allows for
application across various materials, including metals, semiconductors,
and polymers. Additionally, AFM enables non-destructive imaging of
surfaces with atomic or near-atomic resolution, crucial for character-
izing surfaces before and after nanolithography.
Moreover, AFM-based mechanical lithography allows for direct-
write capabilities, facilitating precise removal of material at the nano-
scale. Real-time monitoring during the fabrication process ensures a
better understanding of the physical phenomena involved in the nano-
lithography process, allowing the comparison of experimental data with
theoretical models of m-SPL.
At rst, we focused on the simplest mechanical AFM-based method,
i.e. nanoindentation, with which it is possible to realize a single or arrays
of nanoholes on the substrate surface. Those nanoholes could be trans-
ferred on a more resilient substrate via etching processes or they can act
as a mask for the deposition of metallic nanodisks. Successively, we
described the Pulse-Atomic Force Lithography which is regarded as the
evolution of the nanoindentation method. Indeed, by the overlapping of
single indentation it is possible to pattern 2D and 3D nanostructures
with constant and varying depth proles. After that, we report AFM
Static and Dynamic Plowing lithography techniques, directly developed
from Contact and Semicontact AFM imaging modes, respectively. In
particular, Static Plowing Lithography involves a stationary probe
scratching the material surface to create nanoscale patterns. On the
other hand, Dynamic Plowing Lithography, uses an oscillating probe to
induce controlled deformation and material removal, allowing for pre-
cise nanostructuring. Finally, we focused on the m-SPL coupled with
additional energy sources such as vibration (UVA AFM and Nano-
milling), high temperature (t-SPL), and voltage (Bias-assisted SPL). More
in details, thermal SPL allows for controlled heating to manipulate
material properties. Bias-assisted lithography utilizes voltage applica-
tion for precise modication of surfaces. Ultrasonic vibration in nano-
milling with AFM enables efcient material removal, expanding the
versatility of nanofabrication processes. These combined techniques
offer a synergistic approach, providing greater exibility and control in
creating nanostructures for various applications.
Several key considerations should be made in developing a practical
and concrete use of m-SPL techniques. The most important aspect that
must be taken into account includes ensuring the reliability of the tip for
patterning extensive geometries without wear or breakage, which would
otherwise compromise the correct realization of the nanostructures.
Another concern emerges if the efciency of the single-tip AFM
system to pattern large areas (of the order of cm
2
) is compared to con-
ventional maskless techniques like EBL, FIB, and TPL. Indeed, m-SPL
offers several advantages, such as the low cost of the nanofabrication
L. Vincenti et al.
Materials & Design 243 (2024) 113036
14
Fig. 8. Bias-assisted m-SPL technique. (a) Sketch representation of the Bias-assisted m-SPL and (b) graph depicting the spatial movement of the AFM probe in the x ,
y, and z directions during the Bias-assisted m-SPL procedure. The red arrow in (a) indicates the force applied by the tip during the lithography while the yellow sign
around the AFM tip represents the bias voltage. (c) AFM 3D topography images of the study of nanochannels patterned by changing the electrical pulse parameters
and the tip velocity. At center, nanogroove fabricated by applying a pulse amplitude of −2 V between the tip and the sample, on-time of 50 ns, at a frequency of 0.5
MHz, and a tip velocity of 10
μ
m/s; The other topography images illustrated the effect of changing one parameter with respect to the central one. Reprinted with
permission from Ref. [131]. Copyright © 2010 IOP Publishing Ltd. (d) AFM topography images of the square patterns realized by coupling AFM lithography on the Cu
thin lm surface and the corresponding cross sections. From left to right, the rst tree squares were sculpted with a force equal to 500 nN and the applied voltage was
set to 4, 6, and 8 V while the last tree squares were patterned with an higher force (1000 nm) and a voltage of 4, 6, and 8 V. Reprinted with permission from Ref. [42].
Copyright © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
L. Vincenti et al.
Materials & Design 243 (2024) 113036
15
apparatus, the capability to perform the nanofabrication at ambient
conditions, the nanometric resolution achievable in the nanopatterning
processes, the non-negligible possibility to image the nanostructures
after their nanopatterning with the same instrument, and the reduction
in the post-processing steps. In addition, the level of detail obtained in
the nanostructures presented in this work is difcult to reach with so-
phisticated conventional techniques like EBL and even unlikely with
techniques of easy implementation, such as photolithography and NIL.
Moreover, the development of new m-SPL techniques, the possibility of
coupling the mechanical action of the process with other energy sources,
and their continuous optimization enhance the m-SPL versatility. In this
context, the selection of the most suitable m-SPL technique for a specic
application largely depends on the characteristics of the material to be
patterned and on the desired level of resolution. Moreover, further ad-
vancements in multifunctional materials manipulation may lead to in-
tegrated nanoscale devices. Additionally, the development of systems
using thousands of tips for parallel operations may offer the most
promising path for integrating m-SPL techniques into industrial pro-
cesses. To sum up, improving 3D nanofabrication capabilities and
increasing throughput could expand AFM applications in both scientic
research and industrial elds. Continued research may also uncover new
applications in elds like nanoelectronics, photonics, and biotech-
nology, further broadening the impact of AFM.
CRediT authorship contribution statement
Lorenzo Vincenti: Writing – review & editing, Writing – original
draft, Methodology, Investigation, Conceptualization. Paolo Pelle-
grino: Writing – review & editing, Writing – original draft, Visualiza-
tion. Mariafrancesca Cascione: Writing – review & editing. Valeria De
Matteis: Writing – review & editing, Conceptualization. Isabella
Farella: Writing – review & editing. Fabio Quaranta: Writing – review
& editing. Rosaria Rinaldi: Writing – review & editing, Supervision,
Resources.
Declaration of competing interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Data availability
No data was used for the research described in the article.
Acknowledgments
I. Farella acknowledges funding PNRR MUR project PE0000023-
NQSTI nanced by the European Union – Next Generation EU.
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