![(a) G-Ni-P composite plating, (b) comparison of the mechanical properties of the Ni-P plating and G-Ni-P composite plating [15]. Reprinted from [15], Copyright (2018), with permission from Elsevier.](https://www.researchgate.net/profile/Yupeng-He-8/publication/353034940/figure/fig4/AS:11431281207790099@1701350510335/a-G-Ni-P-composite-plating-b-comparison-of-the-mechanical-properties-of-the-Ni-P_Q320.jpg)
![(a) Spherical microlens array fabricated on Si by wet etching and (b) the SEM image of a single spherical microlens [26]. Reprinted with permission from [26] © The Optical Society.](https://www.researchgate.net/profile/Yupeng-He-8/publication/353034940/figure/fig5/AS:11431281207864583@1701350510543/a-Spherical-microlens-array-fabricated-on-Si-by-wet-etching-and-b-the-SEM-image-of-a_Q320.jpg)
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International Journal of Extreme Manufacturing
Int. J. Extrem. Manuf. 3(2021) 042002 (32pp) https://doi.org/10.1088/2631-7990/ac1159
Topical Review
A review of the techniques for the mold
manufacturing of micro/nanostructures
for precision glass molding
Tianfeng Zhou1,2,∗, Yupeng He1, Tianxing Wang1, Zhanchen Zhu1, Ruzhen Xu1, Qian Yu1,
Bin Zhao1, Wenxiang Zhao1, Peng Liu1,2and Xibin Wang1,2
1School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081,
People’s Republic of China
2Beijing Institute of Technology Chongqing Innovation Center, Chongqing 401120,
People’s Republic of China
E-mail: zhoutf@bit.edu.cn
Received 22 November 2020, revised 8 April 2021
Accepted for publication 4 July 2021
Published 19 July 2021
Abstract
Micro/nanostructured components play an important role in micro-optics and optical
engineering, tribology and surface engineering, and biological and biomedical engineering,
among other elds. Precision glass molding technology is the most efcient method of
manufacturing micro/nanostructured glass components, the premise of which is meld
manufacturing with complementary micro/nanostructures. Numerous mold manufacturing
methods have been developed to fabricate extremely small and high-quality
micro/nanostructures to satisfy the demands of functional micro/nanostructured glass
components for various applications. Moreover, the service performance of the mold should
also be carefully considered. This paper reviews a variety of technologies for manufacturing
micro/nanostructured molds. The authors begin with an introduction of the extreme
requirements of mold materials. The following section provides a detailed survey of the existing
micro/nanostructured mold manufacturing techniques and their corresponding mold materials,
including nonmechanical and mechanical methods. This paper concludes with a detailed
discussion of the authors recent research on nickel-phosphorus (Ni-P) mold manufacturing and
its service performance.
Keywords: precision glass molding, mold manufacturing, micro/nanostructure, mold material,
extreme features
(Some gures may appear in colour only in the online journal)
∗Author to whom any correspondence should be addressed.
Original content from this work may be used under the terms
of the Creative Commons Attribution 3.0 licence. Any fur-
ther distribution of this work must maintain attribution to the author(s) and the
title of the work, journal citation and DOI.
© 2021 The Author(s). Published by IOP Publishing Ltd on behalf of the IMMT
2631-7990/21/042002+32$33.00 1
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
1. Introduction
Due to their signicant functions in hydrophobicity [1–3],
friction reduction [4], and optical reection and diffraction
[5], micro/nanostructures are widely applied in the elds of
optical imaging and sensing, biomedicine, etc. A micro/n-
anostructure array is composed of many geometric units on
the micro/nanometer scale. Commonly used micro/nanoarrays
include lens arrays, columnar arrays, groove arrays, and pyr-
amid arrays, as shown in gure 1. In addition, some other
nonconventional micro/nanoarrays have been designed and
manufactured to meet the needs of uniquely designed func-
tional surfaces [5,6]. micro/nanostructured glass compon-
ents have the advantages of miniaturization, integration, and
being lightweight, which are in line with the development
direction of microsystems. As a result, micro/nanostructured
glass components are widely used in the microsystems.
Microlens array structured glass is used in integrated cam-
eras to improve imaging quality and achieve refocusing and
three-dimensional imaging functions. Beneting from its high
diffraction efciency, micro/nanostructured glass is applied
in optical information processing, structural coloration, anti-
counterfeiting, etc [7–9]. The scale reduction and perform-
ance improvement of these microsystems are largely determ-
ined by the quality of the micro/nanostructured glass compon-
ents, which requires overcoming the challenge of manufactur-
ing micro/nanostructures with the features of both extremely
small size and high quality. Considering the broad application
prospects and extreme features of small scale and high qual-
ity, ultra-precision manufacturing technology for the fabrica-
tion of micro/nanostructured glass components has become a
strategic development eld in many countries.
Micro/nanostructures are produced by generating a series
of geometric units on a surface at the micro/nanometer scale
via certain methods. Attempts are currently underway to
advance the extreme manufacturing ability of these methods
in terms of size, accuracy, consistency, and efciency. Glass
molding has been creatively and innovatively used to ef-
ciently manufacture micro/nanostructure arrays on glass sur-
faces, which is deemed as the best technology to fabricate
micro/nanostructures on glass surfaces. The concept of glass
molding technologies involves inducing the proper pressure
at a high temperature to copy the micro/nanostructure array
from a mold onto a glass. During the molding process, glass
is softened via heating and then solidied via annealing. Glass
molding has the advantages of high forming accuracy, ef-
ciency, good consistency, and low processing costs, and is
therefore suitable for the mass production of micro/nanostruc-
ture arrays. It should be noted that the micro/nanostructure
generated on the glass surface is completely copied from the
mold surface during the molding process, so the precise man-
ufacturing of the micro/nanostructure with extreme features of
small size and high quality on the mold is the premise of glass
molding. Recently, many methods, including both mechanical
and nonmechanical methods, have been rapidly developed to
overcome the challenge of manufacturing small, high qual-
ity micro/nanostructures. These novel methods have been suc-
cessfully applied to mold manufacturing. Etching technology
Figure 1. Typical micro/nanostructures. (a) Microlens array,
(b) groove array, (c) columnar array, and (d) pyramid array.
can achieve material removal at the atomic scale, so the pro-
duced micro/nanostructure molds have high precision. How-
ever, this technology is limited by its low manufacturing ef-
ciency and complex process. Laser technology is also widely
applied in surface engineering. Femtosecond laser-induced
periodic surface structures (FLIPSSs) display high efciency
in micro/nanostructure mold manufacturing. Micro-electrical
discharge machining (µEDM) and electrochemical machining
technologies have also been investigated for micro/nanostruc-
ture manufacturing and are especially dominant in the fabric-
ation of microstructures with complex shapes and high aspect
ratios. In addition, ultra-precision cutting technologies and
micro/nanogrinding technologies have been widely used to
machine micro/nanostructures. These mechanical methods use
diamond cutting tools to remove mold materials at the micro/
nanoscale and fabricate the desired structure [10,11]. Com-
pared with those manufactured by nonmechanical methods,
micro/nanostructure molds machined by mechanical machin-
ing technology can directly reach nanometer precision.
This paper explores the key technologies of mold manu-
facturing. As a typical example of molding technologies, the
precision glass molding (PGM) process is rst introduced and
analyzed, as are the extreme requirements of the mold mater-
ials. Subsequently, the newest nonmechanical technologies,
including etching, laser machining, and electrical discharge
machining (EDM), are introduced and explained in detail.
The newest micro/nanostructure processing methods that use
mechanical methods, including turning, milling, y cutting,
grinding, and lapping, are then explained. Next, compound
technologies that combine conventional mechanical machin-
ing the ultrasonic vibration are also introduced and described
in detail. Finally, the authors’ recent study on the service per-
formance of a nickel-phosphorus (Ni-P) micro/nanostructured
mold is presented in detail.
2. Mold materials for glass molding
Glass molding has become the most effective and ef-
cient method for the high-precision manufacturing of
2
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 2. (a) Photograph of PFLF7-60A molding machine and (b) it is schematic diagram.
Figure 3. The glass molding process.
micro/nanostructures on glass surfaces and is implemented
on a molding machine. For instance, as shown in gure 2, the
PFLF7-60A molding machine (produced by SYS, Japan) con-
sists of a heating module, pressurizing module, and automatic
conveyor. The glass is processed into the desired shape after
undergoing four processes in the molding machine, including
heating and maintaining, pressurizing at high temperature,
annealing at low pressure, and cooling and demolding, as
shown in gure 3. The micro/nanostructured mold can be
divided into the upper mold, lower mold, and sleeve. The ver-
tical distance between the upper and lower molds is aligns
with the designed micro/nanostructures, and it can be con-
trolled by regulating the sleeve height. The glass preform
is placed into the mold core and then transferred into the
mold chamber for heating. A protective gas (such as nitro-
gen) lls the mold chamber to prevent the glass and mold
from oxidizing under the high temperature. When the glass
preform is heated 30 ◦C–40 ◦C above the glass softening-
point, the upper and lower molds are closed and loaded to
compress the glass preform to achieve complete lling and
replication of the micro/nanostructures. The mold temperat-
ure is slowly reduced to approximately 200 ◦C, during which
a small load is still applied. Finally, the mold leaves the
mold chamber and is taken out after being cooled to room
temperature.
The main component of glass is silicon dioxide (SiO2),
which is brittle at normal temperatures but exhibits vis-
coelastic characteristics at high temperatures. The temperature
is subdivided into the strain point (StP), annealing point (AP),
transition point (Tg), yield point (At), and softening point (SP)
according to the expansion coefcient characteristics of glass.
The glass used for molding must have a low transition point,
which can extend the mold life and shorten the processing
cycle [12,13].
According to the mechanism of glass molding, the mold
material has a substantial inuence on the interface condition
between the mold and glass, which also affects the precision of
the fabricated glass components. To suppress the forming error
resulting from high-temperature deformation, a mold material
with low thermal expansion and high-temperature resistance
3
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 4. (a) G-Ni-P composite plating, (b) comparison of the mechanical properties of the Ni-P plating and G-Ni-P composite plating
[15]. Reprinted from [15], Copyright (2018), with permission from Elsevier.
should be selected. In addition, under the action of the heating-
cooling temperature cycle and the mold clamping-releasing
pressure cycle, thermal fatigue and stress fatigue will occur in
the microlens mold, which will cause mold wear and failure.
Therefore, the selection of the mold material and mold manu-
facturing technology is essential to the glass molding process.
The materials used for glass molding must have the follow-
ing characteristics: (a) high hardness and strength at high tem-
peratures, a low thermal expansion coefcient, and excellent
stability of the chemical properties at high temperatures; (b)
good material consistency, meaning that it can be processed
to meet the requirements of an optical-grade surface; (c) inert
adhesion and reaction with glass [13,14].
It is important to note that ideal mold materials with
extremely high hardness, such as silicon carbide (SiC) and
tungsten carbide (WC), usually cannot be micro/nanotextured
via mechanical methods due to serious tool wear and the afn-
ity reaction between the materials and the carbon atoms in the
diamond tool during the cutting process. Therefore, machining
large-area micro/nanostructures requires frequent tool chan-
ging, which inevitably results in errors. Therefore, plating and
developing new mold materials with low hardness are neces-
sary for mold manufacturing. Nickel phosphorus (Ni-P), has a
hardness (500–600 HV) less than that of WC, SiC, and other
super-hard materials. Ni-P can meet the hardness requirements
of glass molding. Ni-P exhibits relatively good cutting per-
formance, which makes it more suitable for single-point dia-
mond cutting and use as an ideal mold material for glass mold-
ing [12,13,15]. In addition, it has been demonstrated that
Ni-based materials have excellent lubrication and antiadhes-
ive properties when used with glass at high temperatures [16].
The plating thickness of Ni-P can reach more than 100 µm.
To improve the mechanical properties of Ni-P plating molds,
graphene and graphene oxide were added and co-deposited
with the Ni-P plating on the substrate, acting as reinforce-
ment phases to generate a composite plating. Figure 4shows
the graphene-nickel-phosphorus (G-Ni-P) plating generated
by Yu et al [15], the Young’s modulus and hardness of which
before and after heat treatment were demonstrated to be higher
than those of Ni-P.
3. Mold manufacturing of micro/nanostructures
via nonmechanical methods
3.1. Micro/nano-etching
Etching is a pattern transfer technology that has been widely
used in semiconductor manufacturing. Patterns can be trans-
ferred from a mask to a substrate by selectively etching an
uncovered part of a layer. Etching processes include wet and
dry etching, such as plasma etching, reactive ion etching, and
ion-beam etching (IBE). Wet etching always takes place in a
liquid environment where chemical solutions can react with
the materials to be etched, thereby generating easily remov-
able reaction products. However, the ‘undercutting’ effect in
wet etching can reduce the precision of the etching pattern.
Thus, dry etching, which can achieve high anisotropy via
plasma-assisted material removal, has rapidly been developed
to achieve high-accuracy pattern transfer. Thus far, etching has
been studied in the fabrication of various microstructures with
unique functions on molds for optical and biology elds.
3.1.1. Wet etching. Wet etching has been studied to obtain
functional surfaces. It can achieve the isotropic etching of
amorphous and polycrystalline materials, which can produce
not only trenches and cavities but also microlens structures on
mold materials, as shown in gure 5. Zhang et al produced
a honeycomb-like textured surface with a pitch of 18 µm on
multi-crystalline Si via a masked wet etching process [17].
Sood et al created deep Ta lm trenches with widths ran-
ging from 10 to 200 µm via hot NaOH/H2O2and KOH/H2O2-
based solutions [18]. By investigating the inuences of various
masks and solutions on the etching process, researchers have
found ways to achieve deep etching and have produced cavities
on the order of hundreds of microns. Chen et al and Tong
et al produced both closely packed hexagonal and rectangular
4
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 5. (a) Spherical microlens array fabricated on Si by wet etching and (b) the SEM image of a single spherical microlens [26].
Reprinted with permission from [26] © The Optical Society.
Figure 6. (a) The microlens array and (b) three-dimensional graph of microlens on Si substrate after IBE etching [37]. Reprinted from [37],
Copyright (2020), with permission from Elsevier.
concave microlens arrays with diameters less than 100 µm
on silica via the femtosecond-laser-enhanced local wet etch-
ing method, in which the femtosecond laser induces craters
on transparent materials, the chemical etching process is then
accelerated in the laser-induced craters, and then the concave
spherical surfaces begin to form [19,20]. For single-crystal
mold materials, such as Si and GaN, the crystal is etched at
various rates in different crystallographic directions due to the
various arrangements of bonds and atoms. Thus, wet aniso-
tropic etching is used to manufacture spatial shapes on these
materials. Based on this, both concave and convex pyramidal
structures [21,22] and microtip arrays [23] have been formed
on Si wafers. Additionally, structures such as hexagonal pyr-
amids and hillocks have been formed on certain crystal planes
of GaN [24] and AlN [25].
3.1.2. Dry etching. Different from wet etching, dry etching
covers plasma-driven chemical reactions and energetic
ion beams aimed at removing the material in a gaseous
environment, which can yield highly anisotropic etch proles
[27]. Some functional surfaces, such as antireective ‘moth-
eye’ surfaces [28], low-reectivity pyramid-like surfaces [29],
and lamellar-patterned surfaces [30], have been investigated
and produced by the dry etching process. By combining dry
etching with advanced mask fabrication techniques, research-
ers have also produced 2D and 3D microstructure arrays on
various mold materials. 2D structures, such as microgrooves
[31], cylindrical arrays [32], and gratings [33] with vertical
walls, have been fabricated by lithography and dry etching pro-
cesses. To obtain 3D structures, researchers have also invest-
igated novel methods by which to rst produce 3D masks and
then transfer them to the substrates via the dry etching process.
Greyscale lithography [34] and thermal scanning probe litho-
graphy [35] have been studied for the fabrication of 3D micro-
structures, such as microlenses, v-grooves, and pyramids. The
direct production of microstructures on photoresist [36] and
metal [37] masks via single-point diamond cutting has been
proposed, and microlens structures have been successfully
produced on Si via dry etching pattern transfer, as shown in
gure 6.
5
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 7. (a) Line-like grooves superimposed on a pre-machined microstructure by the two-beam interference method and (b) the
cross-section prole of the grooves [42]. Reproduced from [42]. © IOP Publishing Ltd. All rights reserved.
To improve the process, researchers have proposed laser-
assisted etching [38]. However, the possible geometries that
can be generated by laser-assisted etching are very limited, and
it is difcult to ensure the surface quality and geometric accur-
acy of lens.
3.2. Laser processing
3.2.1. Conventional laser processing. Due to the high
brightness, directionality, monochromaticity, and coherence
of lasers, laser processing is characterized by high precision
and efciency in the micro/nano-processing of various mater-
ials. Laser processing technologies can be classied accord-
ing to the lasers’ pulse widths, into conventional laser pro-
cessing (with a pulse width greater than 10 ps) or ultrafast
laser processing (with a pulse width less than 10 ps). Since
the duration of the laser pulse exceeds the electron-lattice heat
transfer time, thermal processing is the main mechanism of
conventional laser processing. In contrast, in ultrafast laser
processing, the pulse width of the laser is far less than the
electron-lattice heat transfer time, which means that it is a non-
thermal processing method [39].
In conventional laser processing, laser ablation, multi-
beam interference, and laser-induced periodic surface struc-
tures (LIPSSs) are the primary methods for creating structures.
In laser ablation, high-intensity lasers incrementally ablate the
mold materials to fabricate the desired structures with typ-
ical sizes on the order of 10 µm to 10 mm [40]. However,
conventional laser ablation is a spot-focused process, which
has extremely low efciency for large-area processing. High-
efciency techniques, including line-focused laser machining,
multi-laser interference, and diffraction-based laser microma-
chining, have been proposed [41]. Regarding two-beam inter-
ference, the periodicity of the line-like surface topography is
determined by the laser wavelength λand the angle between
the interfering sub-beams. In addition, as shown in gure 7,
by conducting direct laser interference patterning, line-like
grooves can be superimposed on pre-machined microstruc-
tures, which may be generated by other technologies [42].
LIPSSs were rst investigated via the use of long-pulsed
lasers. LIPSSs are usually regular grooves with a period equi-
valent to the laser wavelength and are oriented perpendicularly
to the laser polarization direction. The generation of these
LIPSSs on metals is attributed to the interference between the
incident laser light and the excited surface plasmon polaritons
(SPPs), which leads to the periodic energy distribution on the
surface [43].
3.2.2. Ultrafast laser processing. An ultrafast laser, which
has a pulse width that is far less than the electron-lattice
heat transfer time, allows the process to be nished before
lattice heating. Due to the advantages of a minimized
heat-affected zone and ultrahigh power, ultrafast laser pro-
cessing enables smaller-scale and higher quality processing of
micro/nanostructures for almost all types of mold materials,
including metals and semiconductors.
Femtosecond lasers are the most advanced due to their
extremely short pulse width (10−15 s). When a femtosecond
laser pulse strikes a metal surface, the electrons initially absorb
the laser’s pulse energy through the inverse bremsstrahlung
mechanism over a skin layer with a thickness of about
10 nm. Since the electron interaction time is usually short,
it is assumed that the heat that excites the electrons occurs
instantaneously. Therefore, the entire nonequilibrium system
in metals is described as constituting two sub-equilibrium sys-
tems, namely the hot electrons and cold lattice. Figure 8shows
a typical experimental setup of direct femtosecond processing
for the creation of micro/nanostructures. Pulses generated by
femtosecond lasers are focused onto a sample mounted on
a computer controlled XY translation stage through a lens.
A focused line can be obtained through a cylindrical lens
[44]. This femtosecond laser setup is suitable for construct-
ing a single spot on the sample when the translation stage
sits stationary, constructing a single line when translating the
sample along the X-axis or Y-axis, or constructing a larger area
when rastering a sample. Regular micro/nanostructures that
can be created by direct femtosecond laser processing are clas-
sied as LIPSSs, nanohole arrays, and nanostructure-textured
microstructures [45].
It has been found that LIPSSs generated using long-pulse
lasers can also be produced by femtosecond laser pulses
on metals [46–48]. Similarly, FLIPSSs are commonly pro-
duced by multi-pulse ablation in a range of laser uences
6
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 8. Experimental setup of the femtosecond laser processing of micro/nanostructures.
Figure 9. Nanostructure-covered FLIPSSs on (a) W [51] and (b) SS [52]. Reprinted gure with permission from [51], Copyright (2009) by
the American Physical Society. Reprinted with permission from [52] © The Optical Society.
slightly above the ablation threshold. FLIPSSs are more
densely covered by nanostructures [45,49], in contrast to
the smooth LIPSSs produced by long-pulse lasers. Figure 9
depicts FLIPSSs induced on W and stainless steel (SS) covered
with ridge and valley nanostructures. In addition, the period d
of these FLIPSSs is signicantly smaller than that of regular
LIPSSs, which attributes to the change in the effective refract-
ive index of the air-metal interface, affecting the propagation
of excited SPPs [45,49]. To improve the manufacturing ef-
ciency of surface texturing, microlens arrays are used to split
the laser beam into several sub-beams so that several rows
of periodic structures can be induced simultaneously [50]. In
addition, as shown in gure 10, via cylindrical focusing and
scanning the femtosecond laser pulses, Sun et al successfully
manufactured large-area gratings on a Si wafer. The manufac-
turing efciency improved by at least two orders of magnitude
as compared to conventional line-scanning.
The period of FLIPSSs can be controlled by changing the
laser wavelength, incidence angle, and laser uence [54–56].
The ablation environment is another factor that affects the
period of fabricated FLIPSSs. Liquids have larger refractive
indexes than air, which signicantly decreases the period of
the FLIPSSs [57]. Currently, observed FLIPSSs can be classi-
ed as either low-spatial-frequency LIPSSs (LSFLs) or high-
spatial-frequency LIPSSs (HSFLs). LSFLs have a period close
to the laser wavelength, which is explained by the classical
interference model. HSFLs have been obtained on semicon-
ductors [58], dielectrics, and metals [59]. HSFLs have a period
much smaller than the laser wavelength and are usually pro-
duced following several hundreds to thousands of laser irradi-
ations with laser uence lower than the single pulse ablation
threshold [60].
Due to the Gaussian beam prole of the laser beam, only
the central part of the irradiated spot can be ablated, thereby
7
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 10. Large-area FLIPSSs created on Si by cylindrical focusing and scanning the femtosecond laser pulses [53]. Reproduced from
[53]. CC BY NC SA 4.0. Copyright © 2017, The Author(s).
Figure 11. (a) Columnar microstructures covered with a nano-secondary structure [45] and (b) nanostructure-textured microgrooves [67].
[45] John Wiley & Sons. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Reprinted from [67], with the permission of AIP
Publishing.
generating nanoholes on mold materials, such as Si. Fur-
thermore, by scanning the laser beam, surface nanomilling
on a metal surface has been proposed via the use of near-
threshold femtosecond laser pulses [61]. By irradiating gold
nanospheres deposited on a Si surface, the plasmonic laser
nano-ablation technique has been proposed and used for the
fabrication of nanoholes in Si [62]. Micro/nanohole arrays can
be fabricated by interferometric femtosecond laser ablation
[63].
Direct femtosecond laser ablation can produce both nano-
structures and hierarchies by varying the laser beam paramet-
ers. Microstructures with some nanostructures were formed in
irradiated areas at a high laser uence with large numbers of
laser shots. Microspikes covered by irregular nanostructures
with a size of 10–50 nm were produced on Si [64]. Columnar
microstructures fabricated on titanium by a femtosecond laser
were covered by nano-secondary structures [45,65], as shown
in gure 11(a).
FLIPSSs provide an effective way to produce
nanostructure-textured microgrooves (hierarchical grooves)
on mold surfaces. By scanning the focused laser beam across
the workpiece surface, a single microgroove or a microgroove
array can be produced on many materials. Nanostructure-
textured microgrooves can be fabricated not only on the mold
surface, as shown in gure 11(b). Both the valleys and ridges
of the generated microgrooves are extensively covered with
a variety of irregular nanostructures. Recently, an efcient
approach was demonstrated to fabricate highly nanograting-
textured lines on Si via combining the chemical etching and
femtosecond laser. First, amorphous-crystalline nanofringes
are fabricated after femtosecond laser scanning over a line on
the Si surface, which creates almost no material removal. In
the subsequent auxiliary chemical etching, nanograting struc-
tures are obtained and form the hierarchical line, as shown in
gure 12 [66].
3.3. Micro-EDM
EDM is capable of machining all concave and convex micro-
structures, including complex 3D structures with high aspect
ratios. It can be used to process conductive materials, regard-
less of their hardness.
Based on EDM technology, µEDM has been proposed and
employed to machine microstructures. The processing mech-
anism of µEDM is the same as that of EDM. This process util-
izes sequential spark energy in the form of the pulse between
the tool and the workpiece, which are all immersed in dielec-
tric uids. However, the erosion rate is much lower than that in
8
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 12. Nanostructure-textured line fabricated on Si by chemical etching-assisted femtosecond laser modication [66]. Reproduced
from [66]. CC BY 4.0. ©2019 Lan Jiang et al, published by De Gruyter, Berlin/Boston.
Figure 13. The manufacturing process of a concave micro-array. [68] 2018, reprinted by permission of the publisher (Taylor & Francis Ltd,
www.tandfonline.com).
conventional EDM because of the small pulse energy involved.
The micromachining performance depends on the melting
point, thermal conductivity, and electrical conductivity of the
materials. A low melting point and electrical conductivity res-
ult in a good micro-machined shape with a low relative wear
rate. High electrical conductivity and a low melting point pro-
duce low surface roughness, high micro-removal rate, and high
discharge energy efciency. Low thermal conductivity leads to
a high aspect ratio and low micro-removal rate [68].
To improve the manufacturing efciency, electrodes with
micro-tip arrays have been extensively applied. He et al fabric-
ated a concave micro-array using an electrode with a microtip
array to investigate the batch micro-machinability of metal-
lic materials, such as die steel cemented carbide, as shown
in gure 13. An electrode with a micro-tip array was gener-
ated by creating a pyramid microtip array on the electrode
surface using a microdiamond grinding wheel [68]. Tong
et al analyzed the processing errors that resulted from the
9
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 14. Illustration of (a) conventional µEDM and (b) R-µEDM.
ofine-fabricated micro-electrode arrays, and then proposed
an array servo-scanning µEDM process with on-machine fab-
ricated micro-electrode arrays [69]. In µEDM, the minimum
machinable size determines the extreme scale of the machined
microstructure and is dened as the material removal per pulse
discharge. To minimize the material removal per pulse dis-
charge, researchers have utilized new electrode materials with
high electrical resistivity. Koyano et al found that the peak dis-
charge current decreases with increasing electrical resistivity
of the tool electrode. During the fabrication of microrod array,
single-crystal Si tool electrodes with high electrical resistivity
can reduce the average diameter of discharge craters generated
on the microrods to approximately 0.4 µm. However, the Si
tool electrode is also characterized by a tool wear ratio higher
than that of copper, which may attribute to the brittleness of
Si [70].
In recent years, reverse micro-electro-discharge machin-
ing (R-µEDM) has been proposed and has become a prom-
ising technology for the fabrication of precise components and
microstructures with high aspect ratios, as shown in gure 14.
R-µEDM is a noncontact thermal micromachining process
that is extensively used to fabricate single and multiple 2.5-D
features with high aspect ratios and different cross-sections,
such as square, circular, and triangular cross-sections. The
feed rate voltage and capacitance are the process parameters
[71].
During the machining of a high-aspect-ratio microprobe
array by µEDM, heat accumulation and heat deformation may
occur in the microprobe array due to the poor debris expul-
sion in such a narrow spark-gap. It has been demonstrated that
the use of ultrasonic vibration assistance is benecial to debris
removal in µEDM [72,73]. It was found that when using vibra-
tion assistance during µEDM, bubbles are quickly driven out
of the machining gap [74] and the processing efciency sig-
nicantly improves [75]. In addition, Lin et al reported that
the quantity of effective discharge of magnetic force assisted
EDM was greater than that of conventional EDM [76], and
the machined structure’s quality and manufacturing efciency
improved with the assistance of the magnetic force. Adding
a rotating magnetic eld perpendicularly to the electric arc
enhances the material removal rate (MRR) as the debris is
expelled from the machining gap [77]. When added tangen-
tially to the electric eld, the pulsating magnetic eld helps
facilitate the transfer of electrons and the degree of ionization,
which improves the MRR and dimensional accuracy. Singh
et al investigated the combined effect of the application of a
magnetic eld and ultrasonic vibrations to the machining zone
in µEDM and found that it resulted in a higher MRR and less
taper for semicircular microfeatures, as well as higher machin-
ing efciency [78].
Although R-µEDM is suitable for the manufacturing of
microrods of various shapes with high aspect ratios, its pro-
cess capability is limited by the difculty of debris ejection,
which causes debris to stick to the electrodes and creates sec-
ondary and higher-order erosion. Therefore, converting this
disadvantageous phenomenon into a benecial process is sig-
nicant. According to the inuence law of the secondary and
higher-order erosion of debris, the electrode shape can be
correspondingly designed to obtain the objective microstruc-
ture. Roy et al studied the relationship between the mater-
ial removed by secondary and higher-order erosion and that
removed by primary erosion during machining. They found
that secondary and higher-order erosion is ve times more
material than primary erosion [79].
The above examples illustrate the principle details and
latest progress of several nonmechanical methods. Table 1
summarizes the processing characteristics and capabil-
ities of nonmechanical methods for micro/nanostructure
manufacturing.
4. Mold manufacturing of micro/nanostructures via
mechanical machining
The previously discussed nonmechanical methods can achieve
the micro/nanoscale manufacturing, but some resistance exists
in the conductivity and magnetism of the materials. The
cross-sections of the micro/nanostructures cannot be precisely
regulated. In contrast, mechanical methods are available for
most workpiece materials, and can achieve micro/nanostruc-
tures with superior geometric freedom and lower surface
roughness.
4.1. Micro/nano-cutting
Ultra-precision cutting technologies via the use of dia-
mond tools have become very important for the manu-
facturing of micro/nanostructured molds, as they can dir-
ectly produce surfaces with nanometer precision without the
need for subsequent processes [80]. Cutting technologies for
micro/nanostructure manufacturing include diamond turning,
diamond milling, y cutting, and vibration-assisted cutting
technologies.
4.1.1. Single-point diamond turning. Single-point dia-
mond turning (SPDT) can manufacture micro/nanostructures
with high machining speed and accuracy. The most basic
10
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Table 1. Characteristics and capabilities of the nonmechanical methods for micro/nanostructure manufacturing.
Methods
Surface
nishes
within ∼
Achievable
accuracy
within ∼
Typical
feature
size range Advantages Limitations
Wet etching 10 nm 1 µm 10 µm–100 µm•Simple process
•Controllable etching rate
•High accuracy
•Chemical contamination
•Orientation dependent
•Undercutting
•Poor repeatability
Dry etching 10 nm 10 nm 10 nm–10 µm•High feature resolution
•Easy to control
•High reproducibility
•Potential radiation damage
•High accuracy
•High cost
•Poor selectively
•Potential radiation damage
Conventional
laser ablation
10 µm 10 µm 10 µm–1000 µm•Automated process
•No liquid chemical waste
•Not limited by materials
•Low accuracy
•Large thermal effects
Femtosecond
laser for
FLIPSSs
100 nm 100 nm 500 nm–1 µm•Low thermal effects
•Extremely high peak power
•Nonlinear effects
•Lower thermal effects
•Low efciency
•High cost
Micro-EDM 10 µm 100 µm 100 µm–1000 µm•Low cost
•Not limited by material
hardness
•Highly polluting
•Low accuracy
•Limited to conductive materials
conguration of the diamond turning machine is based on
only two controlled axes and a spindle. Therefore, rotation-
ally symmetric structures, including Fresnel lens and aspheric
Fresnel lens are the typical micro/nanostructures machined by
turning. During microstructure cutting, the relative trajectory
of the cutting tool to the workpiece is created by coupling the
rotation of the workpiece and the feed motion of the diamond
tool. The cross-section proles of the structures are determ-
ined by both the diamond tool geometry and the modulation of
the infeed depth. Contouring the desired shape by controlling
both linear axes, easily generates more microstructures with
complex shapes [81]. As a typical microstructure with a rota-
tionally symmetric surface, Fresnel lenses can be machined by
coupling the spindle rotation and the feed motion of the X-axis
and Z-axis. Li et al proposed a true circle processing method
with SPDT to promote the machining precision of a Fresnel
lens mold. The arc-curved surface of the Fresnel lens was
machined with B-axis rotation instead of the frequent servo
feed of each axis, which is utilized in the traditional interpola-
tion method [82]. Moreover, the combination of ultraprecision
machining technology and micro-indentation was proposed to
machine high-accuracy microdimples. The proposed method
achieved the array machining of microdimples with complex
cross-sectional prole [83].
To extend the machinability of the diamond turning of more
complex structures, modulated depth-of-cut (DOC) turning
according to the radial and angular positions of the cutting tool
on the surface has been proposed. According to the modulation
frequency, these processes are called slow tool servo (STS)
turning or fast tool servo (FTS) turning. An STS is driven by
a table of the machine tool, while an FTS is driven by a piezo-
electric actuator and the machine tool itself. Normally, an STS
is used for machining surface structures with large amplitudes
and freeform surfaces with high-aspect-ratio [84]. An FTS is
suitable for generating small-amplitude microstructures on at
or axially symmetric surfaces.
STS turning has recently attracted signicant interest
because it is an economical machining method with no extra
machine for tool drive, compared with FTS turning. Unlike
the conventional process in which a single diamond tool is
used to machine one lens at a time, researchers have developed
an innovative diamond tool trajectory that allows the entire
microlens array to be machined in a single operation using an
STS [85]. Kong et al proposed the orthogonal STS (OSTS)
process to machine wavy microstructure patterns on precision
rollers [86], as shown in gure 15.
Zhang et al proposed a path strategy called the grid machin-
ing method to fabricate a compound eye lens (CEL) mold
using STS technology. The off-centered machining congur-
ation was proposed to prevent shape distortion caused by tool
misalignment. This method was also proven to improve the
form accuracy. The machined CEL mold was ultimately used
in molding technologies [87], and it was proved that the CEL
can machined both on planar surface and aspheric surface, as
shown in gure 16. Spherical concave microlens arrays are
usually fabricated on a single-crystal Si wafer for perform-
ance improvement in infrared optics via STS diamond turn-
ing [88]. Hexagonal microlens arrays offer higher optical ef-
ciency than spherical lenses since the tight arrangement of
hexagons makes the microlenses on the surface denser than
the spherical lenses. The tool servo-driven segment turning
method has also been utilized to reduce the dynamic error of
the machine tool induced by lenslet edges during the cutting of
hexagonal lens arrays. A measured peak-to-valley (PV) error
of ∼300 nm and a surface roughness of ∼5 nm Sa were suc-
cessfully achieved [89].
The virtual spindle-based tool servo (VSTS) diamond
turning method has been proposed to generate discontinuously
11
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 15. Comparison of the machining mechanisms of (a) the conventional STS and (b) OSTS [86]. Reprinted from [86],
Copyright (2016), with permission from Elsevier.
Figure 16. Compound eyes on (a) a planar surface and (b) an aspheric surface [90]. Reprinted from [90], Copyright (2013), with permission
from Elsevier.
structured micro-optics arrays. The VSTS is advanced in that
the virtual spindle axis (VSA) can be constructed at any spe-
cied position during VSTS diamond turning via the multiple
translational and rotational servo motions of the machine tool,
as shown in gure 17. Based on the VSTS, microstructure cell
arrays with specic shapes can be generated by FTS or STS
turning by sequentially passing the VSA through the center
of each microstructure cell. These discontinuous micro-optics
arrays can be created on both planar and freeform surfaces
[91].
FTS diamond turning is a promising machining process
for generating microstructure surfaces that is widely used in
the optics industry. Many researchers have focused on the
development of FTS devices to improve their performance.
The bandwidth, stroke, acceleration, stiffness, and accuracy
of FTS devices signicantly inuence the nal quality of
the machined micro/nanostructures [92]. Moreover, the tool
path strategy of FTS diamond turning is the key factor that
inuences the quality of the machined surface. The tool path
generation for the machining of micro/nanostructures can
be described by either analytical description or nonuniform
rational B-spline description. The form error resulting from the
tool nose radius cannot be ignored in FTS turning. Therefore,
a stable compensation strategy for the tool nose radius should
be implemented to optimize the tool path. In addition, because
the dynamics of machine axes affect the surface quality of
micro/nanostructures, they must be controlled to be consist-
ent with the tool path strategy to minimize the form error [93,
94]. With the urgent demand for bulk metallic glasses (BMGs)
with microstructure surfaces in micro/nano-electromechanical
systems, a sinusoidal grid surface has been created on BMGs,
which veries that FTS technology is a very promising method
of fabricating microstructure surfaces on BMGs [95].
Ultraprecision microstructure functional surfaces on hard
and brittle materials (e.g. ceramic and glass) face the chal-
lenges of both a high surface nish and complex surface shapes
due to their tendency to become damaged in the form of
brittle fractures during machining. For highly brittle mater-
ials, ductile-regime machining is the only method by which
to manufacture high precision micro/nanostructures. FTS dia-
mond turning has been utilized to achieve the ductile-regime
machining of these brittle materials, in which plastic deform-
ation and brittle fracture are the main methods of material
removal. Moreover, this method prevents the cracks produced
during material removal from extending into the nished
surface [96].
12
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 17. Illustration of the VSTS [91]. Reprinted from [91], Copyright (2016), with permission from Elsevier.
Ultrasonic vibration-assisted cutting (UVC) was ini-
tially proposed for machining difcult-to-cut materials (e.g.
hardened steel and WC) to overcome the challenges of
tool wear and processing inefciency [97,98]. UVC is
achieved by integrating ultrasonic vibration motion into con-
ventional diamond machining methods [99] (e.g. turning,
milling [100,101], grinding, and planing [102]). Structures are
created after material removal by a tool edge at high speed.
There are typically two types of vibration modes, namely the
reciprocating and elliptical vibration modes. According to the
generation mechanism of the elliptical trajectory of the tool
tip, elliptical vibrators can be categorized as nonresonant and
resonant elliptical vibrators. In a nonresonant vibrator, the
tool is vibrated by two separate piezoelectric actuators (PZT)
arranged with a right angle, and the working frequency is con-
tinuous but cannot be high due to the low stiffness of the mech-
anical structure [97,103–105]. Regarding resonant vibrators,
researchers have synthesized the resonant vibration of either
the longitudinal vibration or bending vibration with a certain
order to generate elliptical vibration [106,107]. The vibration
frequency of the designed device can be greater than 20 kHz by
exciting the actuators. Unlike nonresonant vibrators, the vibra-
tion frequency of the resonant vibration is xed and cannot be
changed.
Figure 18 illustrates the reciprocating UVC method with
vibration in the nominal cutting direction. It should be noted
that the nominal cutting speed is lower than the maximum
vibration speed in the nominal cutting direction, so that the
tool can be separated from the workpiece in each vibration
cycle. This type of UVC results in smaller cutting forces, a
longer tool life, higher cutting stability, and better surface n-
ishing when machining micro/nanostructures on difcult-to-
cut materials [108]. Regarding microstructure machining, this
type of UPC can only be used to fabricate groove-like micro-
structures, as shown in gure 20. However, to avoid the inter-
ference between the tool tip and machined surface when the
tool moves back, the tool vibration direction is inclined with
respect to the nominal cutting direction to prevent chipping the
tool, which can create a vibration mark on the nished surface
[109].
To solve these problems in the reciprocating UVC
method, elliptical vibration cutting (EVC) has been pro-
posed [110–112] and proven to achieve better performance in
machining difcult-to-cut materials [110,111]. As shown in
gure 19, during EVC, the cutting tool is fed in the nominal
cutting direction, and the tool tip is simultaneously modulated
by the vibration in both the nominal cutting and DOC direc-
tions to elliptically vibrate. In each elliptical vibration cycle,
13
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 18. Schematic illustration of the linear vibration cutting
process.
Figure 19. Schematic diagram of the EVC process.
the tool starts to cut the workpiece at time t0and separates
from the workpiece at t1when the tangential direction of the
tool trajectory is parallel to the tool rake face. The workpiece
is removed between time t0and t1. It should be noted that,
compared with the ordinary cutting process, the friction direc-
tion between the tool rake face and the chip is reversed when
the tangent direction of the tool path exceeds the shear dir-
ection of material removal. The reversal of the friction direc-
tion causes an increase in the nominal shear angle, and sig-
nicantly decreases the chip thickness and the average cutting
forces [107,110,113]; this ultimately prevents burr generation
and regenerative chatter in the EVC process [114].
Amplitude controlled EVC has been proposed to fabric-
ate micro/nanostructures for various applications with high
efciency [115–118]. During amplitude controlled EVC, the
DOC can be changed rapidly by conventional FTS tech-
nology, as shown in gure 21. Thus, EVC technology is
equipped with an FTS function [102,119,120]. Amplitude
controlled EVC has successfully fabricated several types of
micro/nano grooves, as shown in gure 22. In grooves [120],
it was also reported that hexagonal dimples, and angle grids,
on hardened steel and WC can also be created by amplitute
controlled EVC [115,120].
The EVC process leaves periodic cusps on the machined
surface due to the overlapping tool trajectory in each vibration
cycle. A slow cutting speed is therefore utilized to improve
the processed surface with a low cusp amplitude. In contrast
to the general EVC process, large cusps left on the surface
are treated as the texture by using a higher nominal cutting
speed and a constant elliptical vibration amplitude, which is
called elliptical vibration texturing (EVT). Figure 23 presents
the cutting process of EVT. Via the EVT method, Guo and
Ehmann fabricated a series of dimples [121,122] and grooves
for hydrophobicity and structural coloration applications
[123,124].
According to the processing mechanism of EVT, one struc-
ture is created during every vibration cycle [125]; the high
frequency of the ultrasonic vibration dramatically improves
the processing efciency of the micro/nanostructure. With the
decrease of the DOC, noncontinuous dimples are fabricated,
as shown in gure 24. In addition, the overlapping of the gen-
erated dimples in a certain direction can create channels [126].
Although the EVT process is highly efcient for micro/nano-
structure manufacturing, it has not been applied to difcult-
to-cut materials due to severe tool wear and the large cutting
force required for these materials.
A novel rotary ultrasonic texturing (RUT) technique has
been developed for the efcient fabrication of precision
micro/nanostructures [127], as shown in gure 25. The ultra-
sonic vibration, feed motion, and rotation are combined to gen-
erate high-frequency periodic changes in the cutting motion,
which are used to fabricate micro/nano-textured surfaces
[127,128]. The diamond tool can vibrate in 3D space, and the
tool locus can be exibly obtained, which enables the genera-
tion of a variety of microstructures.
4.1.2. Micro/nano-milling. In micro/nano-milling processes,
a monocrystalline diamond tool rotates on a spindle and moves
along the surface of the xed workpiece. During milling oper-
ations, the tool rotates along the axis perpendicular to the
workpiece. At least three numerically controlled axes are used
in this process, and micro/nanostructures with almost any
specic cross-section shape, such as microlens arrays, com-
pound eyes, and groove arrays [80,129], can be machined
with optical quality. The nal surface quality of the machined
micro/nanostructures is directly dependent on the corner
radius of the milling tool. A matter of general importance in
the diamond milling of complex structures is the generation
of an optimized tool path [130], and the micro/nano-milling
processing time is also a vital factor. To machine a large
area of microstructures, the required processing time is usu-
ally several days. Therefore, the tool wear and machining ef-
ciency should also be regarded. Wilson et al used a micro-scale
milling tool to fabricate semicircular patterns on a metallic
surface as the mold [131]. Wan et al generated microgrooves
on titanium (Ti) alloy via micro-milling technology, and TiO2
nanotubes were then generated on the machined microgrooves
via anodic oxidation to form a novel hierarchical micro/nano-
topography composed of microgrooves of 40 µm in depth and
TiO2nanotubes of 70 nm in diameter [132].
14
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 20. (a) Micro-trapezoidal structures and (b) micro-Fresnel lenses fabricated on hardened steel by EVC [109]. Reprinted from [109],
Copyright (2016), with permission from Elsevier.
Figure 21. Schematic diagram of the process of amplitude controlled EVC.
Figure 22. Textured grooves on hardened steel created by amplitude controlled EVC. (a) Sinusoidal, (b) ramp, and (c) zigzag [119].
Reprinted from [119], Copyright (2011), with permission from Elsevier.
During micro-milling, the cutter axis is usually inclined
to prevent a cutting point with a speed of 0. The inclined
axis helps improve the machined surface quality. In addition,
to machining micro/nanostructures with various shapes, the
spindle and tool are tilted at inclined angles to create peri-
odic patterns on workpiece surfaces by adjusting the spindle
speed and feed rate. To machine microdimples on a cylindrical
surface, Matsumura and Takahashi used a two-ute ball-end
mill inclined with respect to the tangential direction [133].
Graham et al fabricated microdimple patterns by tilting the
spindle and tool at an inclined angle and developed ef-
cient surface pattern algorithms to generate different dimple
geometries [134].
Researchers have investigated the application of ultra-
precision multi-axis machining technology in the fabrica-
tion of v-groove components. Ultra-precision side milling
15
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 23. Schematic diagram of the EVT process.
Figure 24. Fabricated noncontinuous dimple structures. [126] (2015) Copyright © 2014, Springer-Verlag London. With permission of
Springer.
Figure 25. (a) Principle of the 3D RUT method using a 3D rotary ultrasonic spindle; (b) a textured surface created by RUT [102].
Reproduced from [102]. CC BY 4.0. Copyright © 2017, The Author(s).
(UPSM) has been proposed to generation infrared hybrid
micro-optics in a single-step ductile mode. In UPSM, the
primary surface of the hybrid micro-optics is constructed via
the removal of the workpiece material, and high-frequency
secondary micro/nanostructures are simultaneously formed by
the interference of the diamond tool edge between two neigh-
boring raster cutting trajectories. During the machining pro-
cess, the rotating spindle feeds horizontally in the X-direction,
and the diamond tool intermittently cuts in and out of the
workpiece surface with a specic DOC. In the meantime, the
16
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 26. Schematic diagram of the UPSM approach for hybrid
surfaces with secondary micro/nanostructures [135]. Reprinted with
permission from [135] © The Optical Society.
workpiece performs the transitional servo motions in the Z-
direction, just like the STS, to deterministically generate the
desired primary surface. After nishing one cutting prole,
a step motion along the raster direction (Y-direction) is car-
ried out on the diamond tool so that the entire workpiece can
undergo material removal [135]. Figure 26 presents the schem-
atic diagram of the UPSM process.
Similarly, ultra-precision raster milling (UPRM) has been
proposed to produce v-grooves and provides more freedom to
produce v-groove structures as compared to precision grind-
ing technology [136,137]. There are two possible cutting
strategies in UPRM, namely horizontal and vertical cutting,
as shown in gure 27. During horizontal cutting, the cutting
tool is fed horizontally. After nishing one cutting step, the
diamond tool moves one step vertically. During vertical cut-
ting, the cutting tool is fed in vertically direction while the
diamond tool moves one step horizontally, increasing the man-
ufacturing efciency beyond that of horizontal cutting. Based
on UPRM, a novel single-step machining process for advanced
hierarchical ribs has been proposed [138,139], as shown in
gure 28. Zhao et al investigated the effects of the cutting
conditions on the quality of hierarchical microstructures via
the single-step cutting method using UPRM and found that
large burrs formed with increased cutting depth, feed rate,
and spindle speed; the plastic deformations became serious
with increased cutting depth and feed rate but were reduced
with the decreased spindle speed [140]. Cheng et al fabricated
bare hydrophobic micro-grooved and micro-pillar cyclic olen
copolymer surfaces with highly precise geometries via UPRM
[141].
4.1.3. Fly cutting for micro/nanostructures. Ultra-precision
y cutting (UPFC) is an intermittent cutting process in which
a diamond tool is mounted with a spindle to intermittently
cut a workpiece. Compared with planing technology, UPFC
can be used to fabricate micro/nanostructures with a higher
cutting speed, which produces a surface with higher quality
[142,143]. According to the conguration, UPFC can gener-
ally be divided into radial y cutting (RFC) and end y cutting
(EFC). In RFC, the cutting tool is mounted on the spindle with
the rake face perpendicular to the rotating spindle, while in
EFC, the rake face of the cutting tool is parallel to the spindle
axis. Due to its constant cutting velocity, the UPFC pro-
cess provides a uniform high-quality surface when machining
freeform micro/nanostructured surfaces, microow cells, and
hybrid structural surfaces with submicrometric form errors
and nanometric surface roughness [144–146]. The intermittent
cutting process results in distinctive surface generation mech-
anisms, including intermittent tool-workpiece relative motion,
tool geometry imprinted into a machined surface, and sur-
face material separation and deformation [147]. The cutting
conguration, tool geometry, kinematic and dynamic errors
[148], tool path, workpiece geometry, and material factors
must be carefully chosen according to the specic objective
micro/nanostructure [144,146,149,150].
During the RFC process, the spindle mounted with the
cutting tool rotates at high speeds, and the workpiece can
be fed either axially (ARFC) or horizontally (HRFC) to the
spindle. The feed motion of the workpiece is completed at
a relatively slow speed, and the two motions of the spindle
rotation and feed motion are coupled to form various fea-
tures. The arrangement of the y-cutting setup makes it more
exible for the fabrication of microgrooves [151]. Guo et al
investigated the cutting force model used in microgroove y
cutting and considered the cutting-edge radius of a single crys-
tal diamond tool [152]. This method can be used in the ultra-
precision machining of difcult-to-machine materials, such as
potassium dihydrogen phosphate crystals [153] and single-
crystal Si [154].
Figure 29(a) illustrates the material removal mechanism
during the HRFC process. HRFC can only generate parallel
grooves with cross-sections consistent with the tool geometry.
The shaded area is the shape of the material removed during a
single y cutting rotation. The thickness of the removed mater-
ial rst increases from 0 at the cut-in side to the maximum
value dmax and then decreases to 0 at the cut-out side. The max-
imum thickness of the removed material is given by
dmax =R−(R−Dc)2+R2−(R−Dc)2−f2
,
where Ris the swing radius of the cutting tool, fis the feed rate
per rotation circle, and Dcis the nominal DOC. Figure 29(b)
illustrates grooves machined by RFC by continuously feed-
ing the workpiece in the horizontal direction perpendicular
to the spindle. It is evident that the geometry of the cutting
edge is directly copied onto the cross-section prole of the
fabricated grooves. Nearly any type of groove-like structure is
machinable via the use of shaped cutting tools. Furthermore,
even prismatic features, such as pyramids and triangular pyr-
amid arrays, can be generated by intersecting several of these
17
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 27. Illustrations of (a) the horizontal cutting strategy of UPRM and (b) the vertical cutting strategy of UPRM [137]. Reprinted from
[137], Copyright (2013), with permission from Elsevier.
Figure 28. Hierarchical ribs fabricated by (a) the horizontal cutting strategy of UPRM and (b) the vertical cutting strategy of UPRM.
[138] (2016) Copyright © 2015, Springer-Verlag London. With permission of Springer.
Figure 29. (a) Mechanism of HRFC and schematic diagram of grooves machined by HRFC [143]. Reproduced from [143].
© IOP Publishing Ltd All rights reserved.
18
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 30. Schematic diagrams of (a) the ARFC method and the corresponding grooves [156] and (b) vibration assisted ARFC and the
corresponding two-level structures [5]. [156] (2020) Copyright © 2020, International Society for Nanomanufacturing and Tianjin University
and Springer Nature Singapore Pte Ltd With permission of Springer. Reproduced from [5]. CC BY-NC-ND 4.0. With permission.
grooves [155]. To achieve this, a rotational axis must be moun-
ted on the moving axis so that the workpiece can be aligned in
an angular and lateral position relative to the rotating plane
of the cutting tool [80]. Before a triangular pyramid array is
cut, the tool alignment with respect to the workpiece is very
important, as the groove at the third angle must intersect at
the same point where the grooves at the rst two angles inter-
sect with each other. A deviation of only a few micrometers
will lead to deformation, and, therefore, unusable structures.
The sizes of the workpiece and structure as well as the feed-
ing speed primarily affect the processing time for y cutting.
Generally, the machining of miniaturized prismatic structures
requires a processing time of several days.
ARFC, which is similar to the vertical machining of ras-
ter milling, is a novel method that uses the existing y cutting
device. During ARFC, the workpiece is fed along the spindle
axis. One groove is generated per rotation of the cutting tool,
and grooves are parallelly arrayed while the workpiece is fed
along the spindle axis. Figure 30(a) presents a schematic dia-
gram of the ARFC method and the corresponding grooves gen-
erated on the surface. The grooves have very high aspect ratios,
and the depth of the grooves increases from 0 on both ends to
the maximum in the middle.
In this method, parallel submicron grooves are produced,
which are similar with those created by diffraction grating. On
both sides of the structure, the distance between the grooves
differs from that in the middle, as the cutting depth is low at the
groove tips. ARFC has the advantage of the exible regulation
of groove spacing by controlling the feed rate.
To meet the demands of generating hierarchical struc-
tures for applications in optics [157], tribology and hydro-
phobicity [144,145], and biology [146], modulated y-
cutting technology has been proposed for the machining of
micro/nanostructures. By vibrating the workpiece in the DOC
direction during ARFC, two-level structures composed of
a rst-order geometry feature and second-order submicron
structures can be generated. Figure 30(b) shows the schem-
atic diagram of the vibration assisted ARFC method and the
corresponding two-level structures. The geometric shape of
the rst-order microfeature can easily be controlled by the
processing parameters (e.g. the vibration frequency, amp-
litude, and offset distance). The cross-section of the second-
order submicron grooves is determined only by the tool
geometry. Groove spacing, which is the key factor for func-
tionality, can also be exibly regulated by the feed rate during
processing [5].
19
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 31. (a) Schematic diagram of the EFC method and (b) hierarchical micro/nanostructures machined by EFCS [158]. Reprinted from
[158], Copyright (2015), with permission from Elsevier.
In EFC, because the cutting tool is mounted on the spindle
with the rake face parallel to the spindle axis, a ring groove
is created on the workpiece during each rotation cycle of the
spindle. As shown in gure 31(a), the offset of the diamond
tool with respect to the spindle center is the swing radius of the
cutting tool, which is also equal to the radius of the generated
ring groove. Side-feeding along the surface of the workpiece
causes the cutting to cover the entire surface. Changing the
angle of the workpiece relative to the translational axis can
generate approximately orthogonal intersections of grooves,
thereby creating pyramid structures. EFC is a highly efcient
method of manufacturing micro/nanostructures.
Inspired by the STS/FTS, the end-y-cutting servo (EFCS)
has been proposed for the deterministic generation of hierarch-
ical micro/nanostructures. Translational servo motions along
the Z-axis are used to form primary desired surfaces with
intricate shapes. Similarly to EFC, the secondary nanostruc-
tures in EFCS are generated by actively controlling the tool
mark residuals [158,159]. The hierarchical micro/nanostruc-
tures machined by EFC are shown in gure 31(b).
The DOC of y cutting during the machining of high-
hardness materials is generally several microns, which leads
to low machining efciency and severe tool wear. To over-
come this problem, y cutting has also been combined with
other ultra-precision machining technologies, such as preci-
sion grinding [160] and laser etching, to fabricate complic-
ated structures. Although y-cutting technology can gener-
ate high-quality micro/nanogrooves at high cutting speeds, the
curvature and slope angle of suitable structures remain limited
due to the large swing radius [151,161].
4.2. Micro/nano grinding and lapping
4.2.1. Micro grinding. Due to the enhanced optical per-
formance requirements of molded components and mold-
ing temperature, the development of mold materials aims at
20
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 32. Schematic diagram of the general micro-grinding of
microstructures [168]. Reprinted from [165], Copyright (2020),
with permission from Elsevier.
improving the service life and accuracy of the mold [162,163].
Mold materials, such as SiC, WC [164], and silicon nitride,
are must exhibit good performance in terms of high hardness,
high temperature resistance, good wear resistance, and good
chemical stability. Currently, micro-grinding is also the best
method by which to fabricate microstructures on mold mater-
ials. It has received signicant attention in the eld of micro-
structure mold manufacturing and has been applied to the pro-
cessing of microstructures such as diffractive optical elements,
microgrooves [164–166], microlens arrays, pyramid micro-
structures, and anti-reective moth-eye gratings [167], among
others.
Diamond wheels are commonly utilized for micro-grinding
because they exhibit superior cutting performance when cut-
ting mold materials; they can produce lower surface rough-
ness so that the surface accuracy of the microstructure sur-
face can be ensured. Prole grinding is commonly used to
machine microstructures and indicates that the cross-section
prole of the machined structure is consistent with the wheel
shape [167]. As shown in gure 32, a diamond wheel with the
same shape as the desired microstructures rotates at a very high
speed, and the bonded diamond abrasives act as many micro-
cutters for material removal. Figure 33 presents microgrooves
fabricated by prole micro-grinding on WC. Generally, the
processing accuracy and surface roughness can reach 0.1 and
0.025 µm, respectively.
To fabricate smaller microstructures with more complic-
ated shapes on molds, the microdiamond wheel has been
proposed and utilized in microstructure surface machining.
Aurich et al proposed the concept of superabrasive electro-
plated grinding wheels based on dened grain structures
[168,169]. Because the strength and impact resistance of a
grinding wheel are mainly determined by the bonding agent,
metal bonds are predominantly employed in the fabrica-
tion of superabrasive grinding wheels due to their excellent
formability and high strength compared to resin bonds and
vitried bonds [168,170]. Electroplating and electroless plat-
ing methods can be used to fabricate superabrasive micro-
grinding wheels with metal bonds. The electro-plating method
has been widely used to fabricate micro-grinding wheels due
to its low cost. Onikura et al conducted the fabrication of
micro-cylindrical diamond grinding wheels with diameters
from 100 to 500 µm via the electroplating of diamond grits in a
Watts bath [168,171], and the fabricated microdiamond wheel
was then utilized to fabricate grooves on Tungsten carbide
(WC) and cemented carbide [172], as shown in gure 34.
However, only a single layer of abrasives can be embed-
ded in a metal bond with high residual stress, resulting in the
short life of the grinding wheel [168]. The uniformity of abras-
ives on the grinding wheel is also limited by the distribution
of the current, which may reduce the shape accuracy of the
microwheel [168]. It has been found that electroless plating
has the ability to create uniform coatings on complex shapes
[168,173]. The generated composite platings exhibit excel-
lent functional performance due to the corresponding embed-
ded particles. Composite platings with good wear and abrasion
resistance have been fabricated by embedding hard particles,
such as diamond particles [168]. Compared with the electro-
plating method, the electroless composite plating method can
be used to prepare uniform composite platings with well dis-
tributed abrasive grains for the fabrication of micro-grinding
wheels [168], as shown in gure 35.
Although disc-shaped diamond grinding wheels are not as
exible as diamond pens, they can achieve better surface qual-
ity and higher processing efciency. Moreover, disc-shaped
diamond grinding wheels exhibit higher rigidity and better
wear resistance when processing the microstructure surface.
Because the machining scale is on the order of micrometers,
only a small part with a size of several micrometers is used to
grind the microstructures. Therefore, the grinding wheel eas-
ily becomes worn during grinding, which causes large errors
at the edges and internal corners of the microstructure surface.
In addition, under the same processing conditions, the MRR of
ultra-precision grinding on a microstructure’s surface is lower
than that on surfaces with symmetric shapes, such as spherical
and aspheric surfaces, and the surface roughness after grinding
is also higher.
Ultrasonic vibration assisted grinding (UVG) is achieved
by applying regular and controllable high-frequency vibra-
tion to the abrasive particles in a certain direction (e.g. the
Z-direction) [174]. The introduction of ultrasonic vibration
changes the relative motion between the abrasive particles on
the grinding wheel and the workpiece during the grinding pro-
cess, which alters the movement characteristics of the abrasive
particles. It has been demonstrated that UVG can reduce the
surface roughness of the workpiece and improve the surface
quality during microstructure manufacturing [175]. It can also
reduce the grinding force [176] and further improve the sta-
bility of the cutting system. UVG can reduce grinding wheel
blockage, effectively maintain the sharpness of the abrasive
grains, improve the cutting performance of the grinding wheel,
reduce grinding wheel wear, and improve grinding efciency.
21
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 33. Microgrooves fabricated by micro-grinding.
Figure 34. (a) Electroplated micro-diamond wheel [171] and (b) machined grooves on WC [172]. Reprinted with permission from [171]
© Trans Tech Publications Ltd Reprinted from [172], Copyright (2009), with permission from Elsevier.
Figure 35. Micro-diamond wheel fabricated by electroless composite plating. (a) Tip shape; (b) tip morphology [168]. Reprinted from
[168], Copyright (2020), with permission from Elsevier.
In UVG, the cavitation effect resulting from ultrasonic vibra-
tion emulsies the cutting uid into particles so that the cut-
ting uid can easily inltrate the contact surface between the
grinding wheel and the workpiece. The inltrating cutting uid
helps fully cool and lubricate the surface, thereby preventing
surface burns on the workpiece [174].
4.2.2. Ultra-precision lapping. Lapping is a method of pro-
cessing microstructures via the micro-cutting action of abras-
ives on workpieces. The abrasive is added between the tool and
the workpiece. When pressure is applied, the relative move-
ment of the tool and the workpiece causes a small amount of
cutting so that a very thin metal layer of the surface of the
22
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 36. The schematic of a 2D cross-section of the structured mold fabrication process [177]. Reproduced from [177] © IOP Publishing
Ltd All rights reserved.
Figure 37. Photographs of (a) a microlens array machined on a Si
wafer and (b) the corresponding microlens array on a polymer [177].
Reproduced from [177] © IOP Publishing Ltd All rights reserved.
workpiece is evenly cut off. To improve the machined surface
quality, the material used as the lapping tool (lapping head)
is softer than the workpiece; thus, the abrasive embeds in the
surface of the lapping tool under the effect of pressure.
The lapping process can produce an optically nished sur-
face during Si wafer production; it is widely used for microlens
fabrication. However, it is not suitable for the groove-like
microstructure fabrication. Liu et al proposed a lapping-based
process to create evenly distributed microlens arrays on Si
mold surfaces using steel balls and diamond slurries, as shown
in gure 36. To obtain a high-quality microlens with a highly
accurate geometry and surface nish, the material selection of
the lapping ball and the lapping time were carefully invest-
igated. They fabricated a microcavity with a ∼10 nm surface
roughness in 30 s, which improved both the processing ef-
ciency and surface quality [177]. A 5 ×5 microlens array was
fabricated on a Si wafer, as shown in gure 37.
Liu et al further developed a lapping system for the pre-
cision fabrication of 3D microlens arrays on nonplanar silica
surfaces. With the assistance of the stage movement, the
3D microlens arrays were precisely fabricated on both con-
cave and convex surfaces via the developed experiment setup,
as shown in gure 38. To achieve micropatterns by prop-
erly arranging microlenses on a curved surface, a micro-
wear model was also established to calculate the relation-
ships between the microlens sag height and the lapping para-
meters, such as the downward force, relative sliding velocity,
and lapping time. Specically, two groups of microlens arrays
with different apertures were produced on concave and convex
silica surfaces, respectively [178].
Although the section above introduces many typical
mechanical machining methods for the fabrication of
micro/nanostructured molds, these methods are suitable for
different processing requirements when considering factors
including the micro/nanostructure size, processing accuracy
and environment, because of their different processing prin-
ciples. Table 2shows the characteristics capabilities of these
mechanical methods for micro/nanostructure manufacturing.
5. Service performance of micro/nanostructure
molds in glass molding
5.1. Surface evolution of molds under high temperatures
In micro/nanostructure glass molding, the working temper-
ature of the mold is higher than 500 ◦C. When general
amorphous Ni-P is used as the mold material under high-
temperature conditions, the Ni-P in the mold material under-
goes an amorphous-to-crystalline transition, causing the mold
to deform, which, in turn, affects the precision of the glass
micro/nanostructure molded product.
To investigate the inuence of high-temperature molding
conditions on the accuracy of the micro/nanostructure molds, a
at mold made of amorphous Ni-P material was used for glass
molding. First, the plate mold was polished and marked along
the centerline of the mold. The marked mold was then used
for glass molding at a temperature of 550 ◦C, molding time of
23
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 38. A cross-section sketch of the 3D microlens array lapping process. (a) A lapping ball stuck in the hole on the support and rolled
to lap a micro-cavity on a curved substrate; (b) one micro-cavity produced on the curved surface [178]. Reprinted by permission from
Springer Nature Customer Service Centre GmbH: Springer Nature, Microsystem Technologies [178], Copyright © 2018, Springer-Verlag
GmbH Germany, part of Springer Nature.
Table 2. Characteristics and capabilities of the mechanical methods for micro/nanostructure manufacturing.
Methods
Surface
nishes
Within ∼
Achievable
accuracy
within ∼
Typical
feature
size range Advantages Limitations
SPDT 5 nm 100 nm 10 µm–1000 µm•High-speed cutting
•Low cost
•High accuracy
•Limited to rotationally symmetric
structures
STS 5 nm 100 nm 10 µm–1000 µm•Available for complex
structures
•High accuracy
•Low cost
•Limited to large-amplitude surface
structures
FTS 10 nm 100 nm 1 µm–100 µm•High efciency
•High accuracy
•High response frequency
•Limited to small-amplitude
microstructures on at or axially
symmetric surfaces
EVC 10 nm 100 nm 10 µm–1000 µm•Available for difcult-to-cut
materials
•High accuracy
•Complicated to control the
structure shape
EVT 10 nm 100 nm 100 nm–10 µm•High efciency
•High accuracy
•Powerless to high-hardness
materials
•Complicated to control the
structure shape
UPSM 10 nm 100 µm 100 µm–1000 µm•High accuracy
•Low cost
•Powerless to high-hardness
materials
UPRM /UPFC 10 nm 100 nm 100 nm–1000 µm•High accuracy
•Extreme small size structure
•Low cost
•Powerless to high-hardness
materials
Micro-grinding 100 nm 10 µm 100 µm–1000 µm•Available for high-hard and
brittle materials
•Low cost
•Severe wheel wear
•Potential uid contamination
•Powerless to large-area arrays
Lapping 10 nm 10 µm 500 µm–1000 µm•High surface nishes
•High accuracy
•Rotationally symmetrical structure
•Low efciency
•Limited to brittle materials
10 min, and N2as the shielding gas. The surface shapes before
and after the molding process were measured using a VK-100
laser confocal microscope.
As shown in gure 39, the straight line in the gure
is the surface shape of the at mold before the experi-
ment, and the curve is the deformation of the mold sur-
face measured along the measurement reference line after the
molding process. It was found that although only one mold-
ing cycle was performed, the maximum amount of deforma-
tion of the mold surface caused by the crystallization trans-
formation of the Ni-P material reached to 0.6 µm. During
glass molding, this deformation affects the molding accuracy
of the micro/nanostructures and shortens the service life of the
mold.
24
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 39. Comparison of the morphologies of an amorphous Ni-P at mold before and after molding.
Figure 40. XRD patterns of Ni-P in different phases. (a) Amorphous Ni-P; (b) crystal Ni-P.
To eliminate the mold deformation caused by the crystal-
lization transformation of Ni-P at high temperatures, the Ni-P
material was rst subjected to a high-temperature heat treat-
ment to complete the crystallization transformation, then the
micro/nanostructure was machined on the crystallized Ni-P
material. The heat treatment was conducted with a temperat-
ure of 580 ◦C, treatment time of 2 h, and N2as the shielding
gas. X-ray diffraction (XRD) analysis was carried out for the
Ni-P material before and after treatment, and it was found that
the material was completely transformed into a crystal struc-
ture dominated by the N3P phase after high-temperature heat
treatment, as shown in gure 40.
To investigate the service performance, 500 glass molding
tests were carried out on the crystalline at plate, and the res-
ults are shown in gure 41. Compared with the amorphous Ni-
P materials, crystalline Ni-P material fabricated by heat treat-
ment performs better since its shape deformation is 0.05 µm
after 500 molding processes.
It was found that the crystalline Ni-P material is more
a suitable mold for glass molding since it can achieve
higher accuracy and a longer service life [179]. The spe-
cic processing method is shown in gure 42. First, the
amorphous Ni-P material mold was heat-treated at a high
temperature to complete the crystallization transformation. A
Figure 41. Comparison of the morphologies of the crystal Ni-P at
mold before and after molding.
circular diamond tool was used to cut the mold with sur-
face deformation after crystallization to achieve a surface
roughness Ra< 1 nm. Then the micro/nanostructures were
machined on the at crystalline Ni-P mold surface.
25
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
Figure 42. Eliminating the surface shape error in micro/nanostructure mold machining. (a) Amorphous mold, (b) heat treatment, (c) crystal
mold, (d) at cutting processing, and (e) micro/nanostructure mold cutting.
Figure 43. SEM images of the grooved amorphous Ni-P material
after glass molding.
5.2. Controlling the microdeformation of micro/nanostructure
molds
To investigate the service performance of the micro/nano
structured mold, both the amorphous and crystalline Ni-P
material molds were structured with grooves with period of
20 µm, and the changes of the mold form accuracy before and
after glass molding were compared. For the amorphous Ni-P
microstructure mold, the PV value of the mold increases from
0.37 µm to 0.58 µm after one glass molding cycle, and the
change rate was greater than 50%. In addition, as shown in
gure 43, it was found that the surface roughness Raincreased
25% from 82 nm to 104 nm, which indicates that the amorph-
ous micro/nanostructure mold deteriorated after the molding.
This occurs because the amorphous material undergoes a crys-
tallization transformation at high temperatures, resulting in the
change of the microscopic volume and the deformation of the
processed mold surface. The deformation not only affect the
precision of the mold surface but shorten the mold life.
After 500 pressing tests, the PV values of the crystalline
micro/nanogroove mold were kept at around 0.34. And the sur-
face roughness Raincreased slightly from 2 nm to 2.1 nm, as
shown in gure 44. It demonstrates that the crystal Ni-P mater-
ial shows a better service performance than amorphous Ni-P
under high-temperature molding conditions. A longer service
Figure 44. SEM images of the grooved crystalline Ni-P material
after 500 times glass molding.
life of the mold can also be achieved by using crystalline Ni-P
material for the ultra-precision molding process.
6. Conclusions and outlook
Because micro/nanostructures have numerous special func-
tions and have been widely used in various applications, the
manufacturing of micro/nanostructures on molds plays a vital
role in the mass production of micro/nanostructured compon-
ents. PGM technology contributes to the mass production of
micro/nanostructured components with high efciency and
precision, which requires that the mold materials exhibit good
high-temperature resistance and that the micro/nanostructured
mold exhibit high accuracy. Mold materials should have high
hardness, high-temperature resistance, and inert adhesion and
reaction with glass. In addition, to overcome the challenges of
manufacturing micro/nanostructures on difcult-to-cut molds,
plating generation, the surface treatment of the mold, and the
development of new mold materials with low hardness have
been conducted.
The manufacturing accuracy of the mold affects the nal
accuracy of the molded components; therefore, many meth-
ods, including both mechanical and nonmechanical meth-
ods, have been investigated for the fabrication of various
26
Int. J. Extrem. Manuf. 3(2021) 042002 Topical Review
micro/nanostructure surfaces. Etching is a pattern transfer
technology that can be categorized as either wet or dry etching.
By selectively etching the uncovered part of the layer, the pat-
terns can be transferred from a mask to a substrate. Regarding
laser manufacturing, femtosecond laser technology can induce
periodic solid micro/nanostructures (FLIPSSs) on the surface,
the period of which is determined by the wavelength of the
laser and the processed materials. EDM is a good method for
manufacturing concave and convex microstructures, includ-
ing complex 3D structures with high aspect ratios. Moreover,
µEDM and R-µEDM have been proposed and employed to
remove materials with a low pulse energy. These methods have
been found to improve manufacturing accuracy. Additionally,
the utilization of an array electrode can enhance processing
efciency. Although nonmechanical methods can manufac-
ture micro/nanoscale materials, they are limited in terms of the
materials’ conductivity and magnetism. Moreover, the cross-
sections of the micro/nanostructures cannot be precisely regu-
lated. In contrast, mechanical methods are available for most
workpiece materials and can achieve improved geometric free-
dom and lower surface roughness of micro/nanostructures as
compared to nonmechanical methods.
While only rotationally symmetric structures can be
obtained by conventional turning, STS and FTS extend the
machinability of micro/nanostructures with various shapes.
STS turning is suitable for machining large-amplitude sur-
face structures and high-aspect-ratio freeform surfaces, while
FTS turning can only generate small-amplitude microstruc-
tures on at or axially symmetric surfaces. Suitable machining
strategies have been constructed and optimized to form various
types of micro/nanostructures. Micro/nano-milling is usually
used in the fabrication of microlenses and grooves. Addition-
ally, the application of ultra-precision multi-axis machining
technologies, namely UPSM and raster milling, have demon-
strated for the ability to fabricate hierarchical structures. Fly-
cutting technology has demonstrated substantial advantages in
high-accuracy groove fabrication due to the high-speed cut-
ting process. Change in the feed direction of the y-cutting
tool is used to fabricate different structures; moreover, the pro-
cessing efciency is greatly improved. Ultrasonic vibration
assisted cutting (UVC) is a promising technology for the fab-
rication of micro/nanostructures on difcult-to-cut materials.
In addition, the amplitude-controlled EVC and EVT methods
have been developed and used in the high-efciency fabrica-
tion of micro/nanostructures. Diamond grinding and UVG are
widely applied in the machining of high-hardness materials.
Due to the development of the micro-diamond wheel via both
the electroplating and electroless plating methods, the machin-
able scale can be further reduced. Additionally, because of the
resulting high-quality machined surface, the lapping process
has been used as a method for the ultra-precision machining
of microlenses on both 2D planes and 3D surfaces.
Regarding the service performance of Ni-P microstruc-
ture molds, it has been demonstrated that the mold deform-
ation and surface quality are substantially different before and
after the glass molding process, which is caused by the crys-
tal transition of Ni-P from the amorphous to the crystalline
state. The crystal transition of the micro/nanostructured mold
severely deteriorates the PV value and surface roughness; thus,
the mold life is greatly reduced if amorphous Ni-P is used.
To prevent the mold deformation caused by the crystalliza-
tion transformation of the Ni-P material at high temperatures,
the Ni-P material is rst subjected to high-temperature heat
treatment to complete the crystallization transformation, and
the micro/nanostructure is then machined on the crystal Ni-P
material.
In the future, the techniques to manufacture
micro/nanostructure molds and the extreme features of
micro/nanostructures, including the realization of extremely
small sizes and high quality, will remain the study focus. A
greater expectation of the component performance makes
the large-area processing another extreme feature of the
micro/nanostructured molds, which will require high ef-
ciency and low-cost manufacturing. In addition, due to the
upgrading of mold materials, research on the most suitable
processing methods and techniques will always be a hot spot
in micro/nano mold manufacturing. Meanwhile, the com-
pound technology, such as the combination of etching and
micro/nano cutting, will receive more attention.
Acknowledgments
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.
This work was nancially supported by National Natural
Science Foundation of China (Nos. 51775046 & 51875043
& 52005040), the China Postdoctoral Science Foundation
(No. 2019M660480) and, the Beijing Municipal Natural Sci-
ence Foundation (JQ20014). The authors would also like
to acknowledge support from the Fok Ying-Tong Education
Foundation for Young Teachers in the Higher Education Insti-
tutions of China (No. 151052).
ORCID iD
Tianfeng Zhou https://orcid.org/0000-0003-0846-8647
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